Area of an Ellipse

You know the formula for the area of a circle of radius R. It is PiR².

But what about the formula for the area of an ellipse of semi-major axis of length A and semi-minor axis of length B? For example:

(x²/A²) + (y²/B²) = 1.

The area of such an ellipse is
Area = Pi A * B ,
a very natural generalization of the formula for a circle!