If the length of a diagonal of a square is 18, find the perimeter of the square.

The length of a diagonal of a square is the side*sqrt (2). Let's call one side of the square: s

So we have 18 = s*sqrt (2)

18/sqrt (2) = s

18sqrt (2) / 2 = s

9sqrt (2) = s

Since the perimeter is 4 sides we multiply by 4 to get 36sqrt (2) as the perimeter.