For a square P=4s. That means each side s=p/4 = 40/4=10.
The diagonal of a square makes a right triangle with two sides. The diameter is the hypotenuse (h) of that right triangle.
Pythagorean Theory
so
.
Plug s=10 into that formula.
ft.
If you need help understanding math so you can solve these problems yourself, then one on one online tutoring is the answer ($30/hr). If you need faster solutions with guaranteed detailed answers, then go with personal problem solving ($3.50-$5.50 per problem). Contact me at fcabanski@hotmail.com