1. We have to prove that in a 45°–45°–90° isosceled triangle, the hypotenuse is times the length of each leg.
2. Since, triangle XYZ is a right triangle, the side lengths must satisfy the Pythagorean theorem, which states
But, in isosceles triangle it becomes

3. By combining like terms, we get

4. Now, we will determine the principal square root of both sides of the equation.

5. Dividing both sides of the equation by '2', we get


So, 
6. So the hypothenuse c is
times the length of each leg 'a'.