Question 404480
All sides of a square are equal, so the diagonal of a square splits the square in two right triangles with two sides that are equal.

We use the Pythagorean Theorem to find the length of the side:
 We know that the two sides of the triangle are equal ( because that sides are actually the sides of the square)
 So we have:
 
 a^2 + a^2 = 40^2 ( the diagonal of the square is the hypotenuses)
 2a^2 = 40^2
  a^2 = 800
  a = √800
  a ~ 28.3 cm

 All sides of a square are equal so all sides are 28.3cm