Question
Question: The locus of the poles of the focal chords of a parabola is...
The locus of the poles of the focal chords of a parabola is
A
The directrix of the parabola
B
A focal chord of the parabola
C
The axis of the parabola
D
The tangent at the vertex of the parabola
Answer
The tangent at the vertex of the parabola
Explanation
Solution
Let (x1, y1) be the pole.
Polar of (x1, y1) w.r.to the parabola is S1= 0
yy1 – 2a(x+x1) =0
this passes through the focus (a, 0)
then 0.y1 – 2a(0+x1) = 0
⇒ x1 = 0
∴ locus of (x1 y1) is x = 0 which is the tangent at the vertex.