The maximum area (in sq. units) of a rectangle having its base on the x-axis and its other two vertices on the parabola, y=12−x2 such that the rectangle lies inside the parabola, is:
(A)36
(B)202
(C)32✓ Correct
(D)183
Answer: (C)
Solution
Let the rectangle reach out to x=±a on the axis, with its top corners on the parabola at height 12−a2. Its area is width times height, A=2a(12−a2)=24a−2a3. Setting dadA=24−6a2=0 gives a2=4, so a=2. The area there is 2(2)(12−4)=4×8=32 square units.
JEE Main · 2019 · 12 Jan 2019 · Shift 1 · Single correct option