Question

The line $x - 1 = 0$is the directrix of the parabola $y ^ { 2 } - k x + 8 = 0$. Then one of the values of k is

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

Answer 1

Answer: (C)

4

Answer 2

The parabola is $y ^ { 2 } = 4 \frac { k } { 4 } \left( x - \frac { 8 } { k } \right)$.

Putting . The equation is $Y ^ { 2 } = 4 \cdot \frac { k } { 4 } \cdot X$

∴ The directrix is $X + \frac { k } { 4 } = 0$ i.e.,$x - 1 = 0$is the directrix. So $\frac { 8 } { k } - \frac { k } { 4 } = 1$ $\Rightarrow$ $k = - 8,4$