What is the maximum possible area of the rectangle that is to be inscribed in a semicircle of radius 8?

1 Answer
Aug 29, 2015

64

Explanation:

Firstly, draw the rectangle in the semicircle such that its center lies on the center of the diameter of the circle. Draw two lines from that center to the point where the rectangle intersects the arc of the semicircle.

Denote this length as 8, the radius of the semicircle. Denote the angle between them as theta

Then the area of the triangle bounded by the radii and the rectangle is given by Delta = 1/2 (8)(8) sin theta =32 sin theta

The are of the rectangle is twice the angle of the triangle, A=64 sin theta

(dA)/(d theta) = 64 cos theta

At maximum, (dA)/(d theta) = 0, thus cos theta = 0, theta = pi/2 since within the semicircle, 0 < theta < pi

Therefore A = 64 sin (pi/2) = 64