Description: Sal uses the Pythagorean theorem to find a missing side length in an isosceles triangle. Well the key realization to solve this is to realize that this altitude that they dropped, this is going to form a right angle here and a right angle here and notice, both of these triangles, because this whole thing is an isosceles triangle, we're going to have two angles that are the same. And so now we can use that information and the fact and the Pythagorean Theorem to solve for x. Let's use the Pythagorean Theorem on this right triangle on the right hand side.*Teacher Advisor is 100% free.