Question 274205
If the perimeter of the square is 20 cm's then that means that each side of the square is 5 cm's.
For the diagonal it cuts the square into 2 equal triangles so you can apply pythagorean's theory.
a^2 + b^2 = c^2
a and b are the two sides of the square being 5 cm's and c is the diagonal.
5^2 + 5^2 = c^2
25 + 25 = c^2
50 = c^2
therefore the diagonal is the square root of 50 which is approx. 7