Paragraph: Let $P Q$ be a focal chord of the parabola $y^{2}=4 a x$. The tangents to the parabola at $P$ and…

Paragraph: Let $P Q$ be a focal chord of the parabola $y^{2}=4 a x$. The tangents to the parabola at $P$ and $Q$ meet at a point lying on the line $y=2 x+a, a>0$.
Question: Length of chord $P Q$ is
  1. 7a
  2. 5a
  3. 2a
  4. 3a

Solution

R lies on y = 2x + a
R, y = 2x + a

t - 1 t = - a
       t - 1 t = - 1
t + 1 t 2 = 1 + 4 = 5
      PQ = t + 1 t 2 = 5 a

Asked in: JEE Advanced 2013 (Paper 2)

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