Area of Parallelogram/Square

Theorem
The area of a square equals the product of one of its bases and the associated altitude.

Proof

 * AreaOfParallelogram-Square.png

From Area of Square:
 * $\paren {ABCD} = a^2$

where $a$ is the length of one of the sides of the square.

The altitude of a square is the same as its base.

Hence the result.