The line parallel to the $x$-axis and passing through the intersection of the lines ax $+$ $2 b y+3 b=0$ and…

The line parallel to the $x$-axis and passing through the intersection of the lines ax $+$ $2 b y+3 b=0$ and $b x-2 a y-3 a=0$, where $(a, b) \neq(0,0)$ is
  1. below the $x$-axis at a distance of $\frac{3}{2}$ from it
  2. below the $x$-axis at a distance of $\frac{2}{3}$ from it
  3. above the $x$-axis at a distance of $\frac{3}{-}$ from it
  4. above the $x$-axis at a distance of $\frac{2}{3}$ from it

Solution

$ \begin{aligned} & a x+2 b y+3 b+\lambda(b x-2 a y-3 a)=0 \\ & \Rightarrow(a+b \lambda) x+(2 b-2 a \lambda) y+3 b-3 \lambda a=0 \\ & a+b \lambda=0 \Rightarrow \lambda=-a / b \\ & \Rightarrow a x+2 b y+3 b-\frac{a}{b}(b x-2 a y-3 a)=0 \\ & \Rightarrow a x+2 b y+3 b-a x+\frac{2 a^2}{b} y+\frac{3 a^2}{b}=0 \\ & y\left(2 b+\frac{2 a^2}{b}\right)+3 b+\frac{3 a^2}{b}=0 \\ & y\left(\frac{2 b^2+2 a^2}{b}\right)=-\left(\frac{3 b^2+3 a^2}{b}\right) \\ & y=\frac{-3\left(a^2+b^2\right)}{2\left(b^2+a^2\right)}=\frac{-3}{2} \\ & \end{aligned} $ $y=-\frac{3}{2}$ so it is $3 / 2$ units below $x$-axis

Asked in: JEE Main 2005

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