If one end of a focal chord of the parabola, y2=16x is at ( 1,4 ), then the length of this focal chord is:
(A)25✓ Correct
(B)24
(C)22
(D)20
Answer: (A)
Solution
For y2=16x, 4a=16 so a=4, and a point is (at2,2at). The end (1,4) needs 2at=4, so t=21. The far end of a focal chord carries parameter −t1=−2. The focal-chord length is a(t+t1)2=4(21+2)2=4⋅425=25.
JEE Main · 2019 · 09 Apr 2019 · Shift 1 · Single correct option