Example: A rectangle has its vertices on
the x-axis, on the y-axis, at the origin, and somewhere on the
graph y=4−x2 in the first quadrant. Find the maximum possible area
of such a rectangle. Justify your answer.
Solution:
As shown in the video, our rectangle has width x
and height y, and so has area xy. But y=4−x2, so our area is
A(x)=x(4−x2)=4x−x3.
This turns
our story problem into just finding the maximum value
of A(x) on [0,2]. Since A′(x)=4−3x2, we have a critical number
when 4−3x2=0, or x=2√3. Then
y=4−x2=4−43=83, and the area is A=xy=163√3=16√39.
(The video cuts out a few seconds too soon, but the end of the
calculation is shown to the left.)