Question

If the line $x+1=0$ is the directrix of parabola $y^2-kx+8=0$, then one of the value of $k$ is

• A. $\frac{1}{8}$
• B. $8$
• C. $4$
• D. $\frac{1}{4}$

Answer 1

The correct option is B $8$

Given parabola is
$y^2-kx+8=0$
$\Rightarrow y^2=kx-8=k(x-\frac{8}{k})=4AX$
where $4A=k$
Directrix of the parabola is $X+A=0$
Comparing it with $x+1=0$, we get
$-1=\frac{8}{k}-\frac{k}{4}$
$\Rightarrow -4k=32-k^2$
$\therefore k=8,-4$