Question

If 2x+y+k =0 is a normal to y² = -8x, then k =

• A. –16
• B. –8
• C. –24
• D. 24

Answer 1

Answer: (D)

24

Answer 2

The normal to the parabola y² = 4ax is y+xt = 2at + at³.

∴ The normal to the parabola y² = -8x is y+xt = -4t – 2t³;

Compare this with y+2x = -K

⇒ t = 2 ⇒ -K = -4t – 2t³

⇒ K = 4t + 2t³

= 8 + 16 = 24