Question

If the points (2,-1), (5,k) are conjugate w.r.t. x² = 8y, then k =

• A. 10
• B. 7/2
• C. –12
• D. –2

Answer 1

Answer: (B)

7/2

Answer 2

If (x₁, y₁) and (x₂, y₂) are conjugate points w.r.to a parabola S = 0, then S₁₂ = 0

∴ x₁.x₂ – 2a(y₁+y₂) = 0

⇒ 2x5 – 4 (-1 + k) = 0

⇒ k = 7/2