MCQ
Let $A (-1,1), B (3,4)$ and $C (2,0)$ be given three points. $A$ line $y = mx , m > 0$, intersects lines $AC$ and $BC$ at point $P$ and $Q$ respectively. Let $A _{1}$ and $A _{2}$ be the areas of $\Delta ABC$ and $\Delta PQC$ respectively, such that $A _{1}=3 A _{2}$, then the value of $m$ is equal to:
  • A
    $\frac{4}{15}$
  • $1$
  • C
    $2$
  • D
    $3$

Answer

Correct option: B.
$1$
b
$P \equiv\left( x _{1}, mx _{1}\right)$

$Q \equiv\left( x _{2}, mx _{2}\right)$

$A _{1}=\frac{1}{2}\left|\begin{array}{ccc}3 & 4 & 1 \\ 2 & 0 & 1 \\ -1 & 1 & 1\end{array}\right|=\frac{13}{2}$

$A _{2}=\frac{1}{2}\left|\begin{array}{ccc} x _{1} & mx _{1} & 1 \\ x _{2} & mx _{2} & 1 \\ 2 & 0 & 1\end{array}\right|$

$A _{2}=\frac{1}{2}\left|2\left( mx _{1}- mx _{2}\right)\right|= m \left| x _{1}- x _{2}\right|$

$A _{1}=3 A _{2} \Rightarrow \frac{13}{2}=3 m \left| x _{1}- x _{2}\right|$

$AC : x +3 y =2$

$BC : y =4 x -8$

$P : x +3 y =2$ and $y = mx \Rightarrow x _{1}=\frac{2}{1+3 m }$

$Q : y =4 x -8$ and $y = mx \Rightarrow x _{2}=\frac{8}{4- m }$

$\left| x _{1}- x _{2}\right|=\left|\frac{2}{1+3 m }-\frac{8}{4- m }\right|$

$=\left|\frac{-26 m }{(1+3 m )(4- m )}\right|=\frac{26 m }{(3 m +1)| m -4|}$

$=\frac{26 m }{(3 m +1)(4- m )}$

$\left| x _{1}- x _{2}\right|=\frac{13}{6 m }$

$\Rightarrow \frac{26 m }{(3 m +1)(4- m )}=\frac{13}{6 m }$

$\Rightarrow \quad 12 m ^{2}=-(3 m +1)( m -4)$

$\Rightarrow \quad 12 m ^{2}=-\left(3 m ^{2}-11 m -4\right)$

$\Rightarrow \quad 15 m ^{2}-11 m -4=0$

$\Rightarrow \quad 15 m ^{2}-15 m +4 m -4=0$

$\Rightarrow \quad(15 m +4)( m -1)=0$

$\Rightarrow m =1$

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