
Since, given parabola is symmetric about the y− axis, hence rectangle will also be symmetric about y− axis.
Let one vertex of the rectangle on the x− axis be (α,0), then
Area of rectangle A=2α.(12−α2)
Differentiating both sides with respect to α, we get
⇒dαdA=24−6α2=0⇒α=2,−2
For area to be maximum, put α=2 in the equation for area of the rectangle, we get
Amax=2×2×(12−22)=32