Multiply the radius of a circle by two to yield its diameter. In other words, d = 2r. Multiply the diameter of a circle by pi to yield its circumference. In other words, C = pi*d. Square the radius and multiply it by pi to yield the area. In other words, A = pi*r^2.
Multiply the base of a triangle by its height. Then, divide that value by two to yield the triangle's area. In other words, A = (1/2)b*h. If a triangle is equilateral, then the perimeter of the triangle is equal to three times the length of one side, or P = 3*l.
Multiply the length of a rectangle by its width to yield its area. In other words, A = l * w. Double the value of the length of a rectangle and add it to two times the width to yield the rectangle's perimeter. In other words, A = 2l + 2w.
Square the side of a square to yield its area. In other words, A = l^2. Multiply the side of a square by four to yield its perimeter. In other words, P = 4*l.