D(5, 7), E(4, 3), and F(8, 2) form the vertices of a triangle. What is m∠DEF?
We calculate side lengths by Pythagorean theorem.
DE=√17EF=√17DF=√34
Obviously, DE2+EF2=DF2
So by converse of Pythagorean theorem, m∠DEF = 90o.
