Ok, find the altitude of an isosceles triangle whose base is 10 and whose congruent sides are 9. I found how to find the alt of a right triangle but still don't really comprende.

Help :D

Think of the isoceles triangle as cut in half, so that you have two right angle triangles each with base 5 and hypotenuse of 9. You can now calculate the height of those right angle triangles using the Pythagorean theorem.