What is the length of a side of a square whose diagonal measures 4?

Designate the length by L. From the Pythagorean theorem, the length of the diagonal of a square, which divides the square into two right triangles, each with sides L (and the diagonal as hypotenuse to each right triangle), is square root of 2L2 = 4 (from the problem statement). Squaring both sides separately yields 2L2 = 16 or L = square root of 8 = 2 (square root of 2).