Calculating perimeter and area

The square has perimeter P = 4a and area S = a²; the rectangle has P = 2(a + b) and S = ab. The area of a triangle is S = ½·a·h, of a parallelogram S = a·h, of a trapezoid S = (a + b)/2 · h, where h is the height. For a circle of radius r the circumference is C = 2πr and the area is S = πr². The area of a complex figure can be found by cutting it into simple figures or by subtracting an extra part from a larger figure. All lengths must be in the same unit, and area is expressed in square units.