Question

If length of focal chord of y² = 4ax is l, then angle between axis of parabola and focal chord is

• A. ± sin^–1
• B. ± sin^–1
• C. ± tan^–1
• D. None of these

Answer 1

Answer: (B)

± sin^–1

Answer 2

Let any point on focal chord is P(h, k)

where h = a + r cos a

k = r sin a

\ Put (h, k) in y² = 4ax

Ž r² sin² a = 4a (a + r cos a)

\ r₁ + r₂ =$\frac { - 4 a ^ { 2 } } { \sin ^ { 2 } \alpha }$

Now here length of focal chord is

l = |r₁| + |r₂|

= r₁ –r₂

=

\ l = 4a/sin²a