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.
\(DE = \sqrt{17}\\ EF = \sqrt{17}\\ DF = \sqrt{34}\)
Obviously, \(DE^2 + EF^2 = DF^2\)
So by converse of Pythagorean theorem, m∠DEF = 90o.
