$45\text{-}45\text{-}90$ Triangle

Since these two acute angles are congruent, its legs are also congruent. This implies that the triangle is isosceles. If the legs have the length $\ell ,$ the length of the hypotenuse will be $\sqrt{2}\ell .$

This relation can be deducted from the trigonometric ratios and the known trigonometric values of the notable angles.