The slope of the line touching both the parabolas y2=4x and x2=−32y is
(A)21✓ Correct
(B)23
(C)81
(D)32
Answer: (A)
Solution
A tangent to y2=4x at the point (t2,2t) is yt=x+t2. For this to touch x2=−32y as well, substitute y=tx+t2 to get x2+t32x+32t=0, and demand a repeated root by setting the discriminant to zero: t21024=128t, so t3=8 and t=2. The slope of the common tangent is t1=21.
JEE Main · 2014 · 06 Apr 2014 · Offline · Single correct option