Q231 / 270 · NDA 2023 (Phase 2) · Mathematics

What is the equation of directrix of parabola $y^2=4 b x$, where $b \lt 0$ and $b^2+b-2=0$ ?

  • A
    $x+1=0$
  • Correct answer
    B
    $x-2=0$
  • C
    $x-1=0$
  • D
    $x+2=0$

Solution

Given equation of Parabola $y^2=4 b x$ Since, $b^2+b-2=0 \Rightarrow(b+2)(b-1)=0$ $\Rightarrow \mathrm{b}=-2,1$ So, equation of directrix is $x=-b$ $\begin{aligned} & \Rightarrow \mathrm{x}=-(-2) \Rightarrow \mathrm{x}=+2 \quad(\text { Since, } \mathrm{b} \lt 0) \\ & \Rightarrow \mathrm{x}-2=0 \end{aligned}$

Hence, the correct answer is option: 'B'.

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