Calculus
posted by Stuck .
"A rectangle is inscribed in a semicircle of radius 2 cm. Find the largest area of such a rectangle".
There is a diagram, but I think the question makes it clear enough what is going on. I'm having problems finding a relationship that I can work with.

draw a radius from the centre to a vertex of the rectangle
Let the base of the rectangle be 2x, making the triangle base x, and let the height of the rectangle by y
Area of rectangle = 2xy
but x^2 + y^2 = 4
y = √(4  x^2) or (4x^2)^(1/2)
A = 2x(4x^2)^(1/2)
take the derivative using the product rule,
set it equal to zero and solve for x 
Thank you! Never occurred to me to position the radius like that.