If the sides of a triangle have lengths a, b, and c, and c^2=a^2+b^2, then the triangle is a right triangle.
This tells us that we can use the Pythagorean Theorem in reverse to test if a triangle is right. The hypotenuse c will always have the greatest value. Let's look at the given triangle.
Let's substitute a= 80, b= 39, and c= 89 into a^2+b^2=c^2, and see if they become a true statement.
