1.

Show that the chords of contacts of points on the line 2x - 3y + 4 = 0 with respect to the parabola y2 = 4ax pass through a fixed point.

Answer»

Let P(x1, y1) be a point on the line 2x - 3y + 4 = 0. Therefore

2x1 - 3y1 + 4 = 0 ....(1)

Now, the chord of contact of (x1, y1) with respect to y2 + 4ax is

yy1 - 2a(x + x1) = 0 ....(2)

From Eqs. (1) and (2), we get

yy1 - 2ax - a(3y1 - 4) = 0

y1(y - 3a) 2a(x - 2) = 0  ...(3)

Eq. (3) represents the lines passing through the fixed point which is the intersection of the lines x = 2 and y = 3a. Hence, the fixed point is (2, 3a).



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