I am really stuck with this question:

A rectangle is drawn inside a semicircle of base radius 10cm.

a) Show that the area of the rectangle is 200 sin x cos x cm^2

b) Calculate the maximum value of this area

There is no diagram but there is actually a triangle inside of the rectangle with an angle of x. The base of 10cm is on the adjacent side of the triangle (so at the bottom)

If someone can help that would be very grateful

Thanks