# 2019 Matriculation Examination : Math Paper Solution

MATRICULATION EXAMINATION
DEPARTMENT OF MYANMAR EXAMINATION
MATHEMATICS        Time Allowed: 3 hours

1.(a)      Functions $\displaystyle f$ and $\displaystyle g$ are defined by $\displaystyle f(x)=x+1$, and $\displaystyle g(x)=2x^2-x+3$. Find the values of $\displaystyle x$ which satisfy the equation $\displaystyle (f\circ g)(x)=4x+1$.
(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ f(x)=x+1,\ \ \\\\\ \ \ \ \ g(x)=2{{x}^{2}}-x+3\\\\\ \ \ \ \ \left( {f\circ g} \right)(x)=4x+1\\\\\therefore \ \ \ f\left( {2{{x}^{2}}-x+3} \right)=4x+1\\\\\therefore \ \ \ 2{{x}^{2}}-x+3+1=4x+1\\\\\therefore \ \ \ 2{{x}^{2}}-5x+3=0\\\\\therefore \ \ \ (x-1)(2x-3)=0\\\\\therefore \ \ \ x=1\ (\text{or})\ x=\displaystyle \frac{3}{2}\end{array}$

1.(b)      The expression $\displaystyle 2x^2+5x-3$ leaves a remainder of $\displaystyle 2p^2-3p$ when divided by $\displaystyle 2x-p$. Find the values of $\displaystyle p$.
(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \text{Let}\ f(x)=2{{x}^{2}}+5x-3.\ \\\\\ \ \ \ \ \text{By the problem,}\\\\\ \ \ \ \ f\left( {\displaystyle \frac{p}{2}} \right)=2{{p}^{2}}-3p\\\\\therefore \ \ \ 2{{\left( {\displaystyle \frac{p}{2}} \right)}^{2}}+5\left( {\displaystyle \frac{p}{2}} \right)-3=2{{p}^{2}}-3p\\\\\therefore \ \ \ \displaystyle \frac{{{{p}^{2}}}}{2}+\displaystyle \frac{{5p}}{2}-3=2{{p}^{2}}-3p\\\\\therefore \ \ \ {{p}^{2}}+5p-6=4{{p}^{2}}-6p\\\\\therefore \ \ \ 3{{p}^{2}}-11p+6=0\\\\\therefore \ \ \ (3p-2)(p-3)=0\\\\\therefore \ \ \ p=\displaystyle \frac{2}{3}\ (\text{or})\ p=3\end{array}$

2.(a)      Find and simplify the coefficient of $\displaystyle x^7$ in the expansion of $\displaystyle {{\left( {{{x}^{2}}+\frac{2}{x}} \right)}^{8}},x\ne 0$.
(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \text{In the expansion of}{{\left( {{{x}^{2}}+\displaystyle \frac{2}{x}} \right)}^{8}},x\ne 0\\\\\ \ \ \ \ {{(r+1)}^{{\text{th}}}}\ \text{term}={}^{8}{{C}_{r}}{{\left( {{{x}^{2}}} \right)}^{{8-r}}}{{\left( {\displaystyle \frac{2}{x}} \right)}^{r}}\\\\\therefore \ \ \ {{(r+1)}^{{\text{th}}}}\ \text{term}={}^{8}{{C}_{r}}{{2}^{r}}{{x}^{{16-3r}}}\\\\\therefore \ \ \ \text{For}\ {{x}^{7}},\ 16-3r=7\\\\\therefore \ \ \ r=3\\\\\therefore \ \ \ \text{Coefficient of}\ {{x}^{7}}={}^{8}{{C}_{3}}{{2}^{3}}\\\\\therefore \ \ \ \text{Coefficient of}\ {{x}^{7}}=\displaystyle \frac{{8\times 7\times 6}}{{1\times 2\times 3}}\times 8=448\end{array}$

2.(b)      Find the sum of all even numbers between $\displaystyle 69$ and $\displaystyle149.$
(3 marks)

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 All even numbers between $\displaystyle 69$ and $\displaystyle149.$ are $\displaystyle 70, 72, 74,$ ... $\displaystyle 148$. It is an $\displaystyle A.P$ with $\displaystyle a=70$, $\displaystyle d=2$ and $\displaystyle {{u}_{{_{n}}}}=148$. $\displaystyle \begin{array}{l}\therefore \ \ \ \ a+\left( {n-1} \right)d=148\\\\\therefore \ \ \ \ 70+\left( {n-1} \right)2=148\\\\\therefore \ \ \ \ n=40\\\\\therefore \ \ \ \ \text{Required Sum}\ ={{S}_{{40}}}\\\\\ \ \ \ \ \ {{S}_{n}}=\displaystyle \frac{n}{2}\left( {a+l} \right)\\\\\therefore \ \ \ \ {{S}_{{40}}}=\displaystyle \frac{{40}}{2}\left( {70+148} \right)\\\\\therefore \ \ \ \ {{S}_{{40}}}=4360\end{array}$

3.(a)      The matrices $\displaystyle A=\left( {\begin{array}{*{20}{c}} 2 & 0 \\ 0 & 5 \end{array}} \right)$ and $\displaystyle B=\left( {\begin{array}{*{20}{c}} x & y \\ 0 & z \end{array}} \right)$ are such that $\displaystyle AB=A+B$. Find the values of $\displaystyle x, y$ and $\displaystyle z$.
(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ A=\left( {\begin{array}{*{20}{c}} 2 & 0 \\ 0 & 5 \end{array}} \right),\ B=\left( {\begin{array}{*{20}{c}} x & y \\ 0 & z \end{array}} \right)\\\\\ \ \ \ AB=A+B\,\ \ \left[ {\text{given}} \right]\\\\\therefore \,\ \ \left( {\begin{array}{*{20}{c}} 2 & 0 \\ 0 & 5 \end{array}} \right)\ \left( {\begin{array}{*{20}{c}} x & y \\ 0 & z \end{array}} \right)=\left( {\begin{array}{*{20}{c}} 2 & 0 \\ 0 & 5 \end{array}} \right)+\ \left( {\begin{array}{*{20}{c}} x & y \\ 0 & z \end{array}} \right)\\\\\therefore \,\ \ \left( {\begin{array}{*{20}{c}} {2x+0} & {2y+0} \\ 0 & {0+5z} \end{array}} \right)\ =\left( {\begin{array}{*{20}{c}} {2+x} & y \\ 0 & {5+z} \end{array}} \right)\\\\\therefore \ \ \ \left( {\begin{array}{*{20}{c}} {2x} & {2y} \\ 0 & {5z} \end{array}} \right)\ =\left( {\begin{array}{*{20}{c}} {2+x} & y \\ 0 & {5+z} \end{array}} \right)\\\\\therefore \ \ \ 2x=2+x\Rightarrow x=2\\\\\ \ \ \ \ 2y=y\Rightarrow y=0\\\\\ \ \ \ \ 5z=5+z\Rightarrow z=\displaystyle \frac{5}{4}\end{array}$

3.(b)      A die is thrown. If the probability of getting a number not less than x is $\displaystyle \frac{2}{3}$, find $\displaystyle x$.
(3 marks)

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 Set of possible outcomes = $\ \displaystyle \left\{ {1,2,3,4,5,6} \right\}$ Number of possible outcomes = $\ \displaystyle 6$ $\displaystyle P(\text{a number not less than}\,x)=\frac{2}{3}=\frac{4}{6}$ Number of favourable outcomes = $\ \displaystyle 4$ Since $\displaystyle P(\text{a number not less than}\,3)=\frac{4}{6}$, $\displaystyle \therefore \ \ \ x=3$.

4.(a)      $\displaystyle AT$ and $\displaystyle BT$ are tangents to the circle $\displaystyle ABC$ at $\displaystyle B$. Prove that $\displaystyle \angle BTX=2\angle ACB$.

(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \gamma =\alpha =\beta \ \ \ \ \ \ (\angle \ \text{between tangents and chord}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{=}\angle \ \text{in alternate segments)}\\\\\ \ \ \ \text{But}\ \theta =\alpha +\beta \ \ \ (\text{exterior }\angle \ \text{of a }\vartriangle \\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{=}\ \text{sum of opposite interior }\angle \text{s)}\\\\\therefore \ \ \theta =\gamma +\gamma =2\gamma \\\\\therefore \ \ \angle BTX=2\angle ACB\end{array}$

4.(b)      The coordinates of $\displaystyle A, B$ and $\displaystyle C$ are $\displaystyle (1,0), (4,2)$ and $\displaystyle (5,4)$ respectively. Use vector method to determine the coordinates of $\displaystyle D$ if $\displaystyle ABCD$ is a parallelogram.
(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ A=(1,0),\ B=(4,2),\ C=(5,4)\\\\\ \ \ \ \ \text{Let}\ D=(a,b).\\\\\ \ \ \ \ ABCD\ \text{is a parallelogram}\text{.}\\\\\therefore \ \ \ \overrightarrow{{AD}}=\overrightarrow{{BC}}\\\\\therefore \ \ \ \overrightarrow{{OD}}-\overrightarrow{{OA}}=\overrightarrow{{OC}}-\overrightarrow{{OB}}\\\\\ \ \ \ \ \left( {\begin{array}{*{20}{c}} a \\ b \end{array}} \right)-\left( {\begin{array}{*{20}{c}} 1 \\ 0 \end{array}} \right)=\left( {\begin{array}{*{20}{c}} 5 \\ 4 \end{array}} \right)-\left( {\begin{array}{*{20}{c}} 4 \\ 2 \end{array}} \right)\\\\\therefore \ \ \ \ \left( {\begin{array}{*{20}{c}} {a-1} \\ b \end{array}} \right)=\left( {\begin{array}{*{20}{c}} 1 \\ 2 \end{array}} \right)\\\\\therefore \ \ \ a-1=2\Rightarrow a=2\\\\\ \ \ \ \ b=2\end{array}$

5.(a)      Solve the equation $\displaystyle 2\cos x \sin x = \sin x$ for $\displaystyle 0{}^\circ \le x\le 360{}^\circ$.
(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ 2\cos x\sin x=\sin x,\ 0{}^\circ \le x\le 360{}^\circ \\\\\ \ \ \ \ 2\cos x\sin x-\sin x=0\\\\\ \ \ \ \ \sin x\left( {2\cos x-1} \right)=0\\\\\ \ \ \ \ \sin x=0\ \left( {\text{or}} \right)\ 2\cos x-1=0\\\\\ \ \ \ \ \sin x=0\ \left( {\text{or}} \right)\ \cos x=\displaystyle \frac{1}{2}\\\\(\text{i})\ \ \text{For}\ \sin x=0,\\\\\ \ \ \ \ x=0{}^\circ \ \ \left( {\text{or}} \right)\ \ x=180{}^\circ \ \ \left( {\text{or}} \right)\ \ x=360{}^\circ \\\\(\text{i})\ \ \text{For}\ \cos x=\displaystyle \frac{1}{2},\\\\\ \ \ \ \ x=60{}^\circ \ \ \left( {\text{or}} \right)\ \ x=300{}^\circ \end{array}$

5.(b)      Differentiate $\displaystyle x^3+2x$ with respect to $\displaystyle x$ from the first principle.

(3 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \text{Let}\ y={{x}^{3}}+2x.\\\\\therefore \ \ \ \ y+\delta y={{\left( {x+\delta x} \right)}^{3}}+2\left( {x+\delta x} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ={{x}^{3}}+3{{x}^{2}}\left( {\delta x} \right)+3x{{\left( {\delta x} \right)}^{2}}+{{\left( {\delta x} \right)}^{3}}+2x+2\left( {\delta x} \right)\\\\\therefore \ \ \ \delta y=\ \left( {y+\delta y} \right)-y\\\\\ \ \ \ \ \ \ \ \ \ \ \ =3{{x}^{2}}\left( {\delta x} \right)+3x{{\left( {\delta x} \right)}^{2}}+{{\left( {\delta x} \right)}^{3}}+2\left( {\delta x} \right)\\\\\therefore \ \ \ \displaystyle \frac{{\delta y}}{{\delta x}}=3{{x}^{2}}+3x\left( {\delta x} \right)+{{\left( {\delta x} \right)}^{2}}+2\end{array}$ $\displaystyle \therefore \ \ \ \displaystyle \frac{{dy}}{{dx}}=\underset{{\delta x\to 0}}{\mathop{{\lim }}}\,\displaystyle \frac{{\delta y}}{{\delta x}}$ $\displaystyle \therefore \ \ \ \frac{{dy}}{{dx}}=\underset{{\delta x\to 0}}{\mathop{{\lim }}}\,\left[ {3{{x}^{2}}+3x\left( {\delta x} \right)+{{{\left( {\delta x} \right)}}^{2}}+2} \right]$ $\displaystyle \therefore \ \ \ \frac{{dy}}{{dx}}=3{{x}^{2}}+2$

##### SECTION (B)(Answer any FOUR questions.)

6.(a)      The functions $\displaystyle f$ and $\displaystyle g$ are defined by $\displaystyle f(x)=2x-1$ and $\displaystyle g(x)=4x+3$. Find $\displaystyle \left( {g\circ f} \right)(x)$ and $\displaystyle {{g}^{{-1}}}(x)$ in simplified form. Show also that $\displaystyle {{\left( {g\circ f} \right)}^{{-1}}}(x)=\left( {{{f}^{{-1}}}\circ {{g}^{{-1}}}} \right)(x)$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \ \ f(x)=2x-1,\ \ g(x)=4x+3\\\\\ \ \ \ \ \ \ \ \left( {g\circ f} \right)(x)=g\left( {f(x)} \right)\\\\\therefore \ \ \ \ \ \ \left( {g\circ f} \right)(x)=g\left( {2x-1} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =4\left( {2x-1} \right)+3\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =8x-1\\\\\ \ \ \ \ \ \ \text{Let}\ {{g}^{{-1}}}(x)=y,\ \text{then}\\\\\ \ \ \ \ \ \ \ g(y)=x\\\\\ \ \ \ \ \ \ \ 4y+3=x\\\\\therefore \ \ \ \ \ \ y=\displaystyle \frac{{x-3}}{4}\\\\\therefore \ \ \ \ \ \ {{g}^{{-1}}}(x)=\displaystyle \frac{{x-3}}{4}\\\\\ \ \ \ \ \ \ \text{Let}\ \ {{\left( {g\circ f} \right)}^{{-1}}}(x)=z,\ \text{then}\\\\\ \ \ \ \ \ \ \left( {g\circ f} \right)(z)=x\\\\\ \ \ \ \ \ \ \ 8z-1=x\\\\\therefore \ \ \ \ \ \ z=\displaystyle \frac{{x+1}}{8}\\\\\therefore \ \ \ \ \ \ {{\left( {g\circ f} \right)}^{{-1}}}(x)=\displaystyle \frac{{x+1}}{8}\ \ \ ---(1)\\\\\ \ \ \ \ \ \ \text{Let}\ {{f}^{{-1}}}(x)=w,\ \text{then}\\\\\ \ \ \ \ \ \ \ f(w)=x\\\\\ \ \ \ \ \ \ \ 2w-1=x\\\\\therefore \ \ \ \ \ \ w=\displaystyle \frac{{x+1}}{2}\\\\\therefore \ \ \ \ \ \ {{f}^{{-1}}}(x)=\displaystyle \frac{{x+1}}{2}\\\\\therefore \ \ \ \ \ \left( {{{f}^{{-1}}}\circ {{g}^{{-1}}}} \right)(x)={{f}^{{-1}}}\left( {{{g}^{{-1}}}(x)} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ={{f}^{{-1}}}\left( {\displaystyle \frac{{x-3}}{4}} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =\displaystyle \frac{{\displaystyle \frac{{x-3}}{4}+1}}{2}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =\displaystyle \frac{{x+1}}{8}\,---(2)\\\\\therefore \ \ \ \ \ \ \text{By}\ (1)\ \operatorname{and}\ (2),\ \\\\\ \ \ \ \ \ \ {{\left( {g\circ f} \right)}^{{-1}}}(x)=\left( {{{f}^{{-1}}}\circ {{g}^{{-1}}}} \right)(x)\end{array}$

6.(b)      The expression $\displaystyle ax^3 - x^2 + bx - 1$ leaves the remainders of $\displaystyle - 33$ and $\displaystyle 77$ when divided by $\displaystyle x + 2$ and $\displaystyle x - 3$ respectively. Find the values of $\displaystyle a$ and $\displaystyle b$, and the remainder when divided by $\displaystyle x - 2$.
(5 marks)

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 $\displaystyle \text{Let}\ f(x)=a{{x}^{3}}-{{x}^{2}}+bx-1.$ $\displaystyle f(x)$ leaves the remainders of $\displaystyle - 33$ when divided by $\displaystyle x + 2$ $\displaystyle \begin{array}{l}\therefore \ \ \ f(-2)=-33\\\\\therefore \ \ \ a{{(-2)}^{3}}-{{(-2)}^{2}}+b(-2)-1=-33\\\\\therefore \ \ \ -8a-4-2b-1=-33\ \\\\\therefore \ \ \ 4a+b=14\ \ \ \ ---(1)\end{array}$ Similarly, $\displaystyle f(x)$ leaves the remainders of $\displaystyle 77$ when divided by $\displaystyle x -3$ $\displaystyle \begin{array}{l}\therefore \ \ \ f(3)=77\\\\\therefore \ \ \ a{{(3)}^{3}}-{{(3)}^{2}}+b(3)-1=77\\\\\therefore \ \ \ 27a-9+3b-1=77\ \\\\\therefore \ \ \ 9a+b=29\ \ \ \ ---(2)\ \\\\\ \ \ \ \text{By}\ (2)-(1),\\\\\ \ \ \ 5a=\ 15\\\\\therefore \ \ a=3\\\\\therefore \ \ 4\left( 3 \right)+b=14\ \\\\\therefore \ \ b=2\ \ \\\\\therefore \ \ f(x)=3{{x}^{3}}-{{x}^{2}}+2x-1.\ \\\\\therefore \ \ f(2)=3{{(2)}^{3}}-{{(2)}^{2}}+2(2)-1=23\ \end{array}$ $\displaystyle \therefore \ \ \ f(x)$ leaves the remainders of $\displaystyle 23$ when divided by $\displaystyle x - 2$

7.(a)      A binary operation $\displaystyle \odot$ on $\displaystyle R$ is defined by $\displaystyle x \odot y = (3y - x)^2 - 8y^2$. Show that the binary operation is commutative. Find the possible values of $\displaystyle k$ such that $\displaystyle 2 \odot k = -31$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ x\odot y={{(3y-x)}^{2}}-8{{y}^{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =9{{y}^{2}}-6xy+{{x}^{2}}-8{{y}^{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ={{x}^{2}}-6xy+{{y}^{2}}\\\\\therefore \ \ \ y\odot x={{(3x-y)}^{2}}-8{{x}^{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =9{{x}^{2}}-6xy+{{y}^{2}}-8{{x}^{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ={{x}^{2}}-6xy+{{y}^{2}}\\\\\therefore \ \ \ x\odot y=y\odot x\\\ \end{array}$ Therefore, the binary operation is commutative. $\displaystyle \begin{array}{l}\ \ \ \ \ x\odot y={{x}^{2}}-6xy+{{y}^{2}}\\\\\ \ \ \ \ 2\odot k=-31\\\\\therefore \ \ \ {{2}^{2}}-6(2)(k)+{{k}^{2}}=-31\\\\\therefore \ \ \ {{k}^{2}}-12k+35=0\\\\\therefore \ \ \ (k-7)(k-5)=0\\\\\therefore \ \ \ k=7\ (\text{or})\ k=5\end{array}$

7.(b)      If the coefficients of $\displaystyle x^r$ and $\displaystyle x^{r+2}$ in the expansion of $\displaystyle (1 + x)^{2n}$ are equal, show that $\displaystyle r = n -1$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \ \ {{(r+1)}^{{\text{th}}}}\text{ term in the expansion of}\ {{(1+x)}^{{2n}}}\\\\\ \ \ \ =\ \ {}^{{2n}}{{C}_{r}}{{x}^{r}}\\\\\therefore \ \ \ \ \ \text{Coefficient of }{{x}^{r}}={}^{{2n}}{{C}_{r}}\\\\\ \ \ \ \ \ \ \text{Coefficient of }{{x}^{{r+2}}}={}^{{2n}}{{C}_{{r+2}}}\\\\\ \ \ \ \ \ \ \text{By the problem, }\\\\\ \ \ \ \ \ \ {}^{{2n}}{{C}_{r}}={}^{{2n}}{{C}_{{r+2}}}\\\\\therefore \ \ \ \ \ r=r+2\ \text{which is impossible}\text{.}\\\\\ \ \ \ \ \ \text{But we have }{}^{{2n}}{{C}_{r}}={}^{{2n}}{{C}_{{2n-r}}}.\\\\\therefore \ \ \ \ {}^{{2n}}{{C}_{{2n-r}}}={}^{{2n}}{{C}_{{r+2}}}\\\\\therefore \ \ \ \ 2n-r=r+2\\\\\therefore \ \ \ \ 2r=2n-2\\\\\therefore \ \ \ \ r=n-1\end{array}$

8.(a)      Find the solution set in $\displaystyle R$ of the inequation $\displaystyle x^2-3x+2\le 0$ by algebraic method and illustrate it on the number line.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \ \ {{x}^{2}}-3x+2\le 0\\\\\therefore \ \ \ \ \ \ \left( {x-1} \right)(x-2)\le 0\\\\\therefore \ \ \ \ \ \ \text{There are two possibilities that}\\\\(\text{i})\ \ \ \ x-1\le 0\ \operatorname{and}\ x-2\ge 0\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{or}\\\\(\text{ii})\ \ \ \ x-1\ge 0\ \operatorname{and}\ x-2\le 0\ \\\\\\(\text{i})\ \ \ \ x-1\le 0\ \operatorname{and}\ x-2\ge 0\\\\\therefore \ \ \ \ \ \ x\le 1\ \operatorname{and}\ x\ge 2\end{array}$ 8a-1-2019 $\displaystyle \therefore\ \ \ \$ There is no point on the number line which satifies both conditions. $\displaystyle \begin{array}{l}(\text{ii})\ \ \ x-1\ge 0\ \operatorname{and}\ x-2\le 0\ \\\\\therefore \ \ \ \ \ x\ge 1\ \operatorname{and}\ x\le 2\ \end{array}$ 8a-2-2019 $\displaystyle \therefore \ \ \ \ \text{Solution set =}\left\{ {x|1\le x\le 2} \right\}$ Number Line 8a-3-2019

8.(b)      Find the sum of the first $\displaystyle 12$ terms of the $\displaystyle A.P$. $\displaystyle 44, 40, 36,$ ... . Find also the sum of the terms between the $\displaystyle {{12}^{{\text{th}}}}$ term and the $\displaystyle {{26}^{{\text{th}}}}$ term of that $\displaystyle A.P.$
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ 44,40,36,...\ \text{is an}\ A.P.\\\\\therefore \ \ a=44,\\\\\ \ \ \ d=40-44=-4\\\\\ \ \ \ {{S}_{n}}=\displaystyle \frac{n}{2}\left\{ {2a+(n-1)d} \right\}\\\\\ \ \ \ {{S}_{{12}}}=\displaystyle \frac{{12}}{2}\left\{ {2\left( {44} \right)+(12-1)\left( {-4} \right)} \right\}\\\\\ \ \ \ {{S}_{{12}}}=264\\\\\ \ \ \ \text{Let the required sum be}\ S.\\\\\therefore \ \ S={{u}_{{13}}}+{{u}_{{14}}}+{{u}_{{15}}}+...+{{u}_{{25}}}\\\\\therefore \ \ S=\displaystyle \frac{{13}}{2}\left( {{{u}_{{13}}}+{{u}_{{25}}}} \right)\\\\\ \ \ \ \ \ \ \ \ =\displaystyle \frac{{13}}{2}\left( {a+12d+a+24d} \right)\\\\\ \ \ \ \ \ \ \ \ =13\left( {a+18d} \right)\\\\\ \ \ \ \ \ \ \ \ =13\left[ {44+18\left( {-4} \right)} \right]\\\\\ \ \ \ \ \ \ \ \ =-364\end{array}$

9.(a)      The sum of the first $\displaystyle n$ terms of a certain sequence is given by $\displaystyle {{S}_{{_{n}}}}={{2}^{n}}-1$. Find the first three terms of the sequence and express the $\displaystyle {{n}^{{\text{th}}}}$ term in terms of $\displaystyle n.$
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ {{S}_{{_{n}}}}={{2}^{n}}-1\\\\\therefore \ \ {{S}_{1}}={{2}^{1}}-1=1\\\\\ \ \ \ {{S}_{2}}={{2}^{2}}-1=3\\\\\ \ \ \ {{S}_{3}}={{2}^{3}}-1=7\\\\\therefore \ \ {{u}_{1}}={{S}_{1}}=1\\\\\ \ \ \ {{u}_{2}}={{S}_{2}}-{{S}_{1}}=2\\\\\ \ \ \ {{u}_{3}}={{S}_{3}}-{{S}_{2}}=4\\\\\therefore \ \ \ {{u}_{n}}={{S}_{n}}-{{S}_{{n-1}}}\\\\\ \ \ \ \ \ \ \ \ =\left( {{{2}^{n}}-1} \right)-\left( {{{2}^{{n-1}}}-1} \right)\\\\\ \ \ \ \ \ \ \ \ ={{2}^{n}}-{{2}^{{n-1}}}\\\\\ \ \ \ \ \ \ \ \ ={{2}^{n}}\left( {1-\displaystyle \frac{1}{2}} \right)\\\\\ \ \ \ \ \ \ \ \ =\displaystyle \frac{{{{2}^{n}}}}{2}\\\\\ \ \ \ \ \ \ \ \ ={{2}^{{n-1}}}\end{array}$

9.(b)      Using the definition of inverse matrix, find the inverse of the matrix $\displaystyle \left( {\begin{array}{*{20}{c}} 3 & 1 \\ 2 & 1 \end{array}} \right)$.
(5 marks)

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 Let $\displaystyle A=\left( {\begin{array}{*{20}{c}} 3 & 1 \\ 2 & 1 \end{array}} \right)$ and $\displaystyle {{A}^{{-1}}}=\left( {\begin{array}{*{20}{c}} a & b \\ c & d \end{array}} \right)$ By the definition of the inverse of the matrix, $\displaystyle A{{A}^{{-1}}}=I,\$ where $\displaystyle I$ is a unit matrix of order $\displaystyle 2$. $\displaystyle \begin{array}{l}\therefore \ \ \left( {\begin{array}{*{20}{c}} 3 & 1 \\ 2 & 1 \end{array}} \right)\left( {\begin{array}{*{20}{c}} a & b \\ c & d \end{array}} \right)=\left( {\begin{array}{*{20}{c}} 1 & 0 \\ 0 & 1 \end{array}} \right)\\\\\ \ \ \ \left( {\begin{array}{*{20}{c}} {3a+c} & {3b+d} \\ {2a+c} & {2b+d} \end{array}} \right)=\left( {\begin{array}{*{20}{c}} 1 & 0 \\ 0 & 1 \end{array}} \right)\\\\\therefore \ \ 3a+c=1----(1)\\\\\,\ \ \ 2a+c=0----(2)\\\\\ \ \ (1)-(2)\Rightarrow a=1\\\\\therefore \ \ 2\times 1+c=0\Rightarrow c=-2\\\\\text{Similarly},\\\\\ \ \ \ 3b+d=0----(3)\\\\\,\ \ \ 2b+d=1----(4)\\\\\ \ \ (3)-(4)\Rightarrow b=-1\\\\\therefore \ \ 3\left( {-1} \right)+d=0\Rightarrow d=3\\\\\therefore \ \ {{A}^{{-1}}}=\left( {\begin{array}{*{20}{c}} 1 & {-1} \\ {-2} & 3 \end{array}} \right)\end{array}$

10.(a)    Find the inverse of the matrix $\displaystyle \left( {\begin{array}{*{20}{c}} 5 & 6 \\ 7 & 8 \end{array}} \right)$. Use it to determine the coordinate of the point of intersection of the lines $\displaystyle 5x + 6y=7$ and $\displaystyle 8y + 7x = 10$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \text{Let}\ A=\ \left( {\begin{array}{*{20}{c}} 5 & 6 \\ 7 & 8 \end{array}} \right).\\\\\therefore \ \ \ \ \det A=40-42=-2\ne 0\\\\\therefore \ \ \ \ {{A}^{{-1}}}\ \text{exists}\text{.}\ \\\\\therefore \ \ \ \ {{A}^{{-1}}}=\displaystyle \frac{1}{{\det A}}\left( {\begin{array}{*{20}{c}} 8 & {-6} \\ {-7} & 5 \end{array}} \right)\ \\\\\ \ \ \ \ \ \ \ \ \ \ \ =-\displaystyle \frac{1}{2}\left( {\begin{array}{*{20}{c}} 8 & {-6} \\ {-7} & 5 \end{array}} \right)\\\\\ \ \ \ \ \left. \begin{array}{l}\ 5x+6y=7\\\ 8y+7x=10\end{array} \right\}\ \ \ \ \left( {\text{given}} \right)\\\\\ \ \ \ \ \left. \begin{array}{l}\ 5x+6y=7\\\ 7x+8y=10\end{array} \right\}---(1)\\\\\ \ \ \ \ \text{Transforming into matrix form,}\\\\\ \ \ \ \ \left( {\begin{array}{*{20}{c}} 5 & 6 \\ 7 & 8 \end{array}} \right)\left( {\begin{array}{*{20}{c}} x \\ y \end{array}} \right)\ =\left( {\begin{array}{*{20}{c}} 7 \\ {10} \end{array}} \right)---(2)\\\\\ \ \ \ \ \text{Let}\ X=\ \left( {\begin{array}{*{20}{c}} x \\ y \end{array}} \right)\ \operatorname{and}\ B=\left( {\begin{array}{*{20}{c}} 7 \\ {10} \end{array}} \right).\\\\\ \therefore \ \ \text{The matrix equation becomes,}\\\\\ \ \ \ \ AX=B\\\\\therefore \ \ \ {{A}^{{-1}}}AX={{A}^{{-1}}}B\\\\\ \ \ \ \ IX={{A}^{{-1}}}B\\\\\therefore \ \ \ X={{A}^{{-1}}}B\\\\\therefore \ \ \ \left( {\begin{array}{*{20}{c}} x \\ y \end{array}} \right)=-\displaystyle \frac{1}{2}\left( {\begin{array}{*{20}{c}} 8 & {-6} \\ {-7} & 5 \end{array}} \right)\left( {\begin{array}{*{20}{c}} 7 \\ {10} \end{array}} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ =-\displaystyle \frac{1}{2}\left( {\begin{array}{*{20}{c}} {56-60} \\ {-49+50} \end{array}} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ =-\displaystyle \frac{1}{2}\left( {\begin{array}{*{20}{c}} {-4} \\ 1 \end{array}} \right)\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ =\left( {\begin{array}{*{20}{c}} 2 \\ {-\displaystyle \frac{1}{2}} \end{array}} \right)\\\\\therefore \ \ x=2,\ \ y=-\displaystyle \frac{1}{2}\end{array}$ Therefore, the point of intersection of the two lines is $\displaystyle \left( {2,-\frac{1}{2}} \right)$.

10.(b)    Construct the table of outcomes for rolling two dice.

Find the probability of an outcome in which the score on the first die is less than that on the second die.

Find also the probability that the score on first die is prime and the score on the second is even.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \,\ \ \ \ \ \ \ \ \ \ \ {{\text{2}}^{{\text{nd}}}}\ \text{die}\\ \ {{\text{1}}^{{\text{st}}}}\ \text{die}\ \ \ \begin{array}{|r||l|l|l|c|c|c|c|c|c|} \hline & 1 & 2 & 3 & 4 & 5 & 6 \\ \hline 1 & {(1,1)} & {(1,2)} & {(1,3)} & {(1,4)} & {(1,5)} & {(1,6)} \\ \hline 2 & {(2,1)} & {(2,2)} & {(2,3)} & {(2,4)} & {(2,5)} & {(2,6)} \\ \hline 3 & {(3,1)} & {(3,2)} & {(3,3)} & {(3,4)} & {(3,5)} & {(3,6)} \\ \hline 4 & {(4,1)} & {(4,2)} & {(4,3)} & {(4,4)} & {(4,5)} & {(4,6)} \\ \hline 5 & {(5,1)} & {(5,2)} & {(5,3)} & {(5,4)} & {(5,5)} & {(5,6)} \\ \hline 6 & {(6,1)} & {(6,2)} & {(6,3)} & {(6,4)} & {(6,5)} & {(6,6)} \\ \hline\end{array}\end{array}$ $\displaystyle \therefore \ \ \$ Number of possible outcomes = $\displaystyle 36$        Set of favourable outcomes for the score in which the score on the first die is less than that on the second die = $\displaystyle \begin{array}{l}\{(1,2),(1,3),(1,4),(1,5),(1,6),\\\ (2,3),(2,4),(2,5),(2,6),(3,4),\\\ (3,5),(3,6),(4,5),(4,6),(5,6)\}\end{array}$ $\displaystyle \therefore \ \ \$ Number of favourable outcomes = $\displaystyle 15$ $\displaystyle \therefore \ P(\text{score on }{{1}^{{\text{st}}}}\text{ die } < \text{score on }{{2}^{{\text{nd}}}}\text{ die) }=\displaystyle \frac{{15}}{{36}}=\displaystyle \frac{5}{{12}}$        Set of favourable outcomes for the score in which the score on first die is prime and the score on the second is even =$\displaystyle \begin{array}{l}\{(2,2),(2,4),(2,6),(3,2),(3,4),\\\ (3,6),(5,2),(5,4),(5,6)\}\end{array}$ $\displaystyle \therefore \ \ \$ Number of favourable outcomes = $\displaystyle 9$ $\displaystyle \begin{array}{l}\therefore \ \ \ \ P(\text{score on }{{1}^{{\text{st}}}}\text{ die is prime and score on }{{2}^{{\text{nd}}}}\ \text{is even)}\\\text{ }\\\ \ \ =\displaystyle \frac{9}{{36}}\\\\\ \ \ =\displaystyle \frac{1}{4}\end{array}$

##### SECTION (C)(Answer any THREE questions.)

11.(a)    $\displaystyle PT$ is a tangent and $\displaystyle PQR$ is a secant to a circle. A circle with $\displaystyle T$ as centre and radius $\displaystyle TQ$ meets $\displaystyle QR$ again at $\displaystyle S$. Prove that $\displaystyle \angle RTS=\angle RPT$
(5 marks)

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 11(a)2019 $\displaystyle \begin{array}{l}\ \ \ \text{In}\ \Delta PTQ,\\\\\ \ \ \alpha =\gamma ~+\angle RPT\\\\\ \ \ \beta =\angle R~+\angle RTS\\\\\ \ \ \text{In}\ \odot T,TQ=TS\ \left( {\text{radii}} \right)\\\\\therefore \ \alpha =\ \beta \\\\\therefore \ \gamma ~+\angle RPT=\ \angle R~+\angle RTS\\\\\ \ \ \text{Since}\ \gamma =\angle R,\ \ (\angle \ \text{between tangent and chord}\\\ \ \ \ \ \ \ \,\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =\angle \ \text{in alternate segments)}\\\\\ \ \angle RTS=\angle RPT\end{array}$

11.(b)    In the diagram, $\displaystyle P$ is the point on $\displaystyle AC$ such that $\displaystyle AP=3PC$, $\displaystyle R$ is the point on $\displaystyle BP$ such that $\displaystyle BR=2RP$ and $\displaystyle QR\parallel AC$. Given that $\displaystyle \alpha \left( {\Delta BPA} \right)=36\ \text{c}{{\text{m}}^{\text{2}}}$, calculate $\displaystyle \alpha \left( {\Delta BPC} \right)$ and $\displaystyle \alpha \left( {\Delta BRQ} \right)$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ AP=3PC\\\\\ \ \ BR=2RP\\\\\ \ \ QR\parallel AC\\\\\ \ \ \alpha (\Delta BPA)=36\ \text{c}{{\text{m}}^{2}}\\\\\ \ \ \text{Since }\Delta BPA\ \operatorname{and}\ \Delta BPC\ \\\ \ \ \text{have the same altitude,}\\\\\,\ \ \displaystyle \frac{{\alpha \left( {\Delta BPC} \right)}}{{\alpha \left( {\Delta BPA} \right)}}=\displaystyle \frac{{PC}}{{AP}}\\\\\therefore \displaystyle \frac{{\alpha \left( {\Delta BPC} \right)}}{{36}}=\displaystyle \frac{{PC}}{{3PC}}\\\\\therefore \alpha \left( {\Delta BPC} \right)=12\ \text{c}{{\text{m}}^{2}}\\\\\ \ \ \text{Since }QR\parallel AC,\\\\\ \ \ \Delta BRQ\sim \Delta BPA\\\\\therefore \displaystyle \frac{{\alpha \left( {\Delta BRQ} \right)}}{{\alpha \left( {\Delta BPA} \right)}}=\displaystyle \frac{{B{{R}^{2}}}}{{B{{P}^{2}}}}\\\\\therefore \displaystyle \frac{{\alpha \left( {\Delta BRQ} \right)}}{{\alpha \left( {\Delta BPA} \right)}}=\displaystyle \frac{{B{{R}^{2}}}}{{{{{(BR+RP)}}^{2}}}}\\\\\therefore \displaystyle \frac{{\alpha \left( {\Delta BRQ} \right)}}{{\alpha \left( {\Delta BPA} \right)}}=\displaystyle \frac{{{{{\left( {2RP} \right)}}^{2}}}}{{{{{(2RP+RP)}}^{2}}}}\\\ \ \ \left[ {\because BR=2RP} \right]\\\\\therefore \displaystyle \frac{{\alpha \left( {\Delta BRQ} \right)}}{{36}}=\displaystyle \frac{4}{9}\\\\\therefore \alpha \left( {\Delta BRQ} \right)=16\ \text{c}{{\text{m}}^{2}}\ \ \text{ }\end{array}$

12.(a)    Prove that the quadrilateral formed by producing the bisectors of the interior angles of any quadrilateral is cyclic.
(5 marks)

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 12(a)2019 $\displaystyle \begin{array}{l}\text{Given}\ :\ \text{Quadrilateral }ABCD,\\\ \ \ \ \ \ \ \ \ \ \ AP,BP,CR\ \operatorname{and}\ DR\\\ \ \ \ \ \ \ \ \ \ \ \text{are interior angle bisectors}\\\ \ \ \ \ \ \ \ \ \ \ \text{of respective vertices}\text{.}\\\\\text{To Prove : }PQRS\ \text{is cyclic}\text{.}\\\\\text{Proof}\ :\ \ \ \ AP,BP,CR\ \operatorname{and}\ DR\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{are interior angle bisectors}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{of respective vertices}\text{.}\\\\\ \ \ \ \ \ \ \ \ \therefore \ \ \ {{\alpha }_{1}}={{\alpha }_{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ {{\beta }_{1}}={{\beta }_{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ {{\gamma }_{1}}={{\gamma }_{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ {{\delta }_{1}}={{\delta }_{2}}\\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ ABCD\ \text{is a}\ \text{quadrilateral}\text{.}\\\\\ \ \ \ \ \ \ \ \ \therefore \ \ \ {{\alpha }_{1}}+{{\alpha }_{2}}+\ {{\beta }_{1}}+{{\beta }_{2}}+{{\gamma }_{1}}+{{\gamma }_{2}}+\ {{\delta }_{1}}+{{\delta }_{2}}=360{}^\circ \\\\\ \ \ \ \ \ \ \ \ \therefore \ \ \ 2{{\alpha }_{1}}+2{{\beta }_{1}}+2{{\gamma }_{1}}+2{{\delta }_{1}}=360{}^\circ \\\\\ \ \ \ \ \ \ \ \ \therefore \ \ \ {{\alpha }_{1}}+{{\beta }_{1}}+{{\gamma }_{1}}+{{\delta }_{1}}=180{}^\circ \\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{In}\ \Delta ABP,\angle P+{{\alpha }_{1}}+{{\beta }_{1}}=180{}^\circ \\\\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{In}\ \Delta CRD,\angle R+{{\gamma }_{1}}+{{\delta }_{1}}=180{}^\circ \\\\\ \ \ \ \ \ \ \ \therefore \ \ \ \angle P+\angle R+{{\alpha }_{1}}+{{\beta }_{1}}+{{\gamma }_{1}}+{{\delta }_{1}}=360{}^\circ \\\\\ \ \ \ \ \ \ \ \therefore \ \ \ \angle P+\angle R+180{}^\circ =360{}^\circ \\\\\ \ \ \ \ \ \ \ \therefore \ \ \ \angle P+\angle R=180{}^\circ \\\\\ \ \ \ \ \ \ \ \therefore \ \ \ PQRS\ \text{is cyclic}\text{.}\ \ \ \ \ \ \ \end{array}$

12.(b)    If $\displaystyle \alpha +\beta +\gamma =180{}^\circ$, prove that $\displaystyle \tan \frac{\alpha }{2}\tan \frac{\beta }{2}+\tan \frac{\beta }{2}\tan \frac{\gamma }{2}+\tan \frac{\alpha }{2}\tan \frac{\gamma }{2}=1$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \alpha +\beta +\gamma =180{}^\circ \\\\\therefore \ \ \ \displaystyle \frac{{\alpha +\beta +\gamma }}{2}=90{}^\circ \\\\\therefore \ \ \ \displaystyle \frac{\alpha }{2}+\displaystyle \frac{\beta }{2}+\displaystyle \frac{\gamma }{2}=90{}^\circ \\\\\therefore \ \ \ \displaystyle \frac{\alpha }{2}+\displaystyle \frac{\beta }{2}=90{}^\circ -\displaystyle \frac{\gamma }{2}\\\\\therefore \ \ \tan \left( {\ \displaystyle \frac{\alpha }{2}+\displaystyle \frac{\beta }{2}} \right)=\tan \left( {90{}^\circ -\displaystyle \frac{\gamma }{2}} \right)\\\\\therefore \ \ \displaystyle \frac{{\tan \left( {\displaystyle \frac{\alpha }{2}} \right)+\tan \left( {\displaystyle \frac{\beta }{2}} \right)}}{{1-\tan \left( {\displaystyle \frac{\alpha }{2}} \right)\tan \left( {\displaystyle \frac{\beta }{2}} \right)}}=\cot \displaystyle \frac{\gamma }{2}\\\\\therefore \ \ \displaystyle \frac{{\tan \left( {\displaystyle \frac{\alpha }{2}} \right)+\tan \left( {\displaystyle \frac{\beta }{2}} \right)}}{{1-\tan \left( {\displaystyle \frac{\alpha }{2}} \right)\tan \left( {\displaystyle \frac{\beta }{2}} \right)}}=\displaystyle \frac{1}{{\tan \displaystyle \frac{\gamma }{2}}}\\\\\therefore \ \ \tan \displaystyle \frac{\beta }{2}\tan \displaystyle \frac{\gamma }{2}+\tan \displaystyle \frac{\alpha }{2}\tan \displaystyle \frac{\gamma }{2}=1-\tan \displaystyle \frac{\alpha }{2}\tan \displaystyle \frac{\beta }{2}\\\\\therefore \ \ \ \tan \displaystyle \frac{\alpha }{2}\tan \displaystyle \frac{\beta }{2}+\tan \displaystyle \frac{\beta }{2}\tan \displaystyle \frac{\gamma }{2}+\tan \displaystyle \frac{\alpha }{2}\tan \displaystyle \frac{\gamma }{2}=1\end{array}$ ========================================= $\displaystyle \alpha=0{}^\circ,\beta=0{}^\circ, \gamma=180{}^\circ$ $\displaystyle \alpha=0{}^\circ,\beta=180{}^\circ, \gamma=0{}^\circ$ $\displaystyle \alpha=180{}^\circ,\beta=0{}^\circ, \gamma=0{}^\circ$ ျဖစ္ရပ္အတြက္ အဓိပၸာယ္ မသတ္မွတ္ႏိုင္ပါ။ Credit : Dr. Aung Kyaw                  ဆရာ ေသာ္ဇင္ထြန္း

13.(a)    In $\displaystyle \Delta ABC$, if $\displaystyle \angle B=\angle A+15{}^\circ$, $\displaystyle \angle C=\angle B+15{}^\circ$ and $\displaystyle BC=6$, find $\displaystyle AC$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \text{In}\ \Delta ABC,\ BC=6\\\\\ \ \ \angle B=\angle A+15{}^\circ \\\\\ \ \ \angle C=\angle B+15{}^\circ \\\\\therefore \ \ \angle C=\angle A+15{}^\circ +15{}^\circ \\\\\ \ \ \ \ \ \ \ \ =\angle A+30{}^\circ \\\\\ \ \ \text{Since}\,\angle A+\angle B+\angle C=180{}^\circ ,\\\\\ \ \ \angle A+\angle A+15{}^\circ +\angle A+30{}^\circ =180{}^\circ \\\\\therefore \ 3\angle A=135{}^\circ \\\\\therefore \ \angle A=45{}^\circ \\\\\therefore \ \angle B=60{}^\circ \ \operatorname{and}\ \angle C=75{}^\circ \\\\\ \ \ \text{By the law of sines,}\\\\\ \ \ \displaystyle \frac{{AC}}{{\sin B}}=\displaystyle \frac{{BC}}{{\sin A}}\\\\\therefore \ \ AC=\displaystyle \frac{{BC\sin B}}{{\sin A}}\\\\\therefore \ \ AC=\displaystyle \frac{{BC\sin 60{}^\circ }}{{\sin 45{}^\circ }}\\\\\therefore \ \ AC=6\times \displaystyle \frac{{\sqrt{3}}}{2}\times \sqrt{2}=3\sqrt{6}\end{array}$

13.(b)    If $\displaystyle y = \ln (\sin^3 2x)$, then prove that If $\displaystyle 3(\frac{{d}^{2}y}{d{x}^{2}}) + (\frac{dy}{dx})^2+36=0$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ y=\ln ({{\sin }^{3}}2x)\ \ \\\ \\\ \ \ \ \ =\ln {{(\sin 2x)}^{3}}\\\\\ \ \ \ \ =3\ln (\sin 2x)\end{array}$ $\displaystyle \ \ \ \frac{{dy}}{{dx}}=\frac{3}{{\sin 2x}}\cdot \frac{d}{{dx}}(\sin 2x)$ $\displaystyle \ \ \ \ \ \ \ \ \ =\frac{3}{{\sin 2x}}\cdot \cos 2x\cdot \frac{d}{{dx}}(2x)$ $\displaystyle \ \ \ \ \ \ \ \ \ =\frac{{6\cos 2x}}{{\sin 2x}}$ $\displaystyle \ \ \ \ \ \ \ \ \ =6\cot 2x$ $\displaystyle \ \ \ \frac{{{{d}^{2}}y}}{{d{{x}^{2}}}}=6(-{{\operatorname{cosec}}^{2}}2x)\frac{d}{{dx}}(2x)$ $\displaystyle \ \ \ \ \ \ \ \ \ \ \ =-12{{\operatorname{cosec}}^{2}}2x$ $\displaystyle \therefore 3(\frac{{{{d}^{2}}y}}{{d{{x}^{2}}}})+{{(\frac{{dy}}{{dx}})}^{2}}+36$ $\displaystyle \begin{array}{l}=3(-12{{\operatorname{cosec}}^{2}}2x)+{{(6\cot 2x)}^{2}}+36\\\\=36(-{{\operatorname{cosec}}^{2}}2x+{{\cot }^{2}}2x+1)\\\\=36(-{{\operatorname{cosec}}^{2}}2x+{{\operatorname{cosec}}^{2}}2x)\ \ \ \ \left[ {1+{{{\cot }}^{2}}2x={{{\operatorname{cosec}}}^{2}}2x} \right]\\\\=0\end{array}$

14.(a)    In the quadrilateral $\displaystyle ABCD$, $\displaystyle M$ and $\displaystyle N$ are the midpoints of $\displaystyle AC$ and $\displaystyle BD$ respectively, prove that $\displaystyle \overrightarrow{{AB}}+\ \overrightarrow{{CB}}+\ \overrightarrow{{AD}}+\ \overrightarrow{{CD}}=4\ \overrightarrow{{MN}}$.
(5 marks)

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 $\displaystyle \begin{array}{l}\ \ \ \ \ \ \text{Let }O\text{ be the origin}\text{.}\\\\\therefore \ \ \ \ \overrightarrow{{AB}}+\ \overrightarrow{{CB}}+\ \overrightarrow{{AD}}+\ \overrightarrow{{CD}}\\\\\ \ \ =\ \overrightarrow{{OB}}-\overrightarrow{{OA}}+\ \overrightarrow{{OB}}-\overrightarrow{{OC}}+\ \overrightarrow{{OD}}-\overrightarrow{{OA}}+\ \overrightarrow{{OD}}-\overrightarrow{{OC}}\\\\\ \ \ =2\left( {\overrightarrow{{OB}}+\overrightarrow{{OD}}-\overrightarrow{{OA}}-\overrightarrow{{OC}}} \right)\ \\\\\ \ \ \ \ \ \text{Since }M\text{ and }N\text{ are the midpoints of }AC\text{ and }BD\text{.}\\\\\ \ \ \ \ \ \overrightarrow{{OM}}=\displaystyle \frac{1}{2}\ \left( {\overrightarrow{{OA}}+\ \overrightarrow{{OC}}} \right)\\\\\ \ \ \ \ \ \overrightarrow{{ON}}=\displaystyle \frac{1}{2}\ \left( {\overrightarrow{{OB}}+\ \overrightarrow{{OD}}} \right)\\\\\therefore \ \ \ \ \ \overrightarrow{{MN}}=\ \overrightarrow{{ON}}-\overrightarrow{{OM}}\\\\\,\ \ \ \ \ \ \ \ \ \ \ \ \ \ =\displaystyle \frac{1}{2}\left( {\overrightarrow{{OB}}+\overrightarrow{{OD}}-\overrightarrow{{OA}}-\overrightarrow{{OC}}} \right)\\\text{ }\\\therefore \ \ \ \ \ 4\overrightarrow{{MN}}=2\left( {\overrightarrow{{OB}}+\overrightarrow{{OD}}-\overrightarrow{{OA}}-\overrightarrow{{OC}}} \right)\\\\\therefore \ \ \ \ \overrightarrow{{AB}}+\ \overrightarrow{{CB}}+\ \overrightarrow{{AD}}+\ \overrightarrow{{CD}}=4\overrightarrow{{MN}}\end{array}$

14.(b)    Find the normals to the curve $\displaystyle xy+2x-y=0$ that are parallel to the line $\displaystyle 2x + y = 0$.
(5 marks)

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 14(b)2019 ပံုဆြဲရန္မလိုပါ $\displaystyle \begin{array}{l}\ \ \ \ \ \ \text{Curve}\ \text{: }xy+2x-y=0\\\\\ \ \ \ \ \ \text{Differentiate with respect to }x.\\\\\ \ \ \ \ \ x\displaystyle \frac{{dy}}{{dx}}+y+2-\displaystyle \frac{{dy}}{{dx}}=0\\\\\therefore \ \ \ \ \displaystyle \frac{{dy}}{{dx}}=\displaystyle \frac{{2+y}}{{1-x}}\\\\\therefore \ \ \ \ \text{gradient of tangent to the curve}=\displaystyle \frac{{2+y}}{{1-x}}\\\\\therefore \ \ \ \ \text{gradient of normal to the curve}=-\displaystyle \frac{1}{{\displaystyle \frac{{dy}}{{dx}}}}=\displaystyle \frac{{x-1}}{{y+2}}\\\ \ \ \ \ \ \ \\\ \ \ \ \ \ \text{Line}\ \text{: }2x+y=0\Rightarrow y=-2x\\\\\therefore \ \ \ \ \text{gradient of line = }-2\ \\\\\ \ \ \ \ \ \text{Since normals }\parallel \ \text{given line}\\\\\ \ \ \ \ \ \text{gradient of normal}=\text{gradient of line}\text{.}\\\\\therefore \ \ \ \ \displaystyle \frac{{x-1}}{{y+2}}=-2\\\\\therefore \ \ \ \ x-1=-2y-4\\\\\therefore \ \ \ \ x=-2y-3\\\\\ \ \ \ \ \ \text{Substituting }x=-2y-3\\\ \ \ \ \ \text{in curve equation,}\\\\\ \ \ \ \ \ (-2y-3)y+2(-2y-3)-y=0\\\ \\\therefore \ \ \ \ {{y}^{2}}+4y+3=0\\\\\therefore \ \ \ \ (y+3)(y+1)=0\\\\\therefore \ \ \ \ y=-3\ (\text{or})\ y=-1\\\\\therefore \ \ \ \ \text{When }y=-3,x=3\\\\\ \ \ \ \ \text{When }y=-1,x=-1\\\\\ \ \ \ \ \text{The equation of normal line at}\\\ \ \ \ \ \ \left( {-1,-1} \right)\ \text{is}\\\\\ \ \ \ \ y+1=-2\left( {x+1} \right)\\\\\therefore \ \ \ \ 2x+y+3=0\\\\\text{ }\ \ \ \ \ \text{The equation of normal line at}\\\ \ \ \ \ \ \left( {3,-3} \right)\ \text{is}\\\\\ \ \ \ \ y+3=-2\left( {x-3} \right)\\\\\therefore \ \ \ \ 2x+y-3=0\end{array}$