Square: s2

The area of a square is the length of a side squared.

Rectangle: L*W

The area of a rectangle is the length multiplied by the width.

Circle: pi*r2

The area of a circle is pi (about 3.14) times the radius squared.

Triangle: 0.5*b*h

The area of a triangle is one half the base times the height.

Trapezoid: 0.5*(b1+b2)*h

The area of a trapezoid is one half the sum of the bases times the height.