The Pythagorean Theorem states that in a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. It can be used to determine if a triangle is right, and to calculate unknown sides. For example, if one side is 35m, another is 12m, and the hypotenuse is 37m, squaring and summing each side according to the theorem (35^2 + 12^2 = 37^2) shows it is a right triangle. It can also be used to solve for missing sides, like finding a hypotenuse of 57.63m when the other two sides are 45m and 36m.