You can find the area of a figure by dividing it into rectangles and half rectangles. This even works to find the (approximate) area of a circle. You know the area of a rectangle: .
It is handy to memorize the following area formulas:
By moving either point or point parallel with the base the area does not change. This is called Cavalieri's principle.
The circumference of a circle with radius
The area of a circle with radius r is: .