A = bh is the area of a rectangle, and also the area of a parallelogram. Any rhombus is also a parallelogram, so you can also use this formula for any rhombus. This helps explain why A = bh shows up three times to account for the three different types of shapes: rectangle, parallelogram, rhombus
A = (1/2)*bh is the area of a triangle which can be written as A = 0.5*b*h
The last formula, which is A = [ (b1+b2)/2 ] *h is the area of a trapezoid. The sides b1 and b2 are the parallel bases
note: the height is always perpendicular to the base
