The Pythagorean Theorem

In a right triangle, the square of the hypotenuse is equal to the sum of
the squares of the legs.

c²= a²+ b²

Theorem:

If the square of one side of a triangle is equal to the sum of the
squares of the other sides then the triangle is a right triangle.

Theorem:

If the square of the longest side of a triangle is greater than the sum
of the squares of the other two sides then the triangle is an obtuse
triangle.

Theorem:

If the square of the longest side of a triangle is less than the sum of
the squares of the other two sides then the triangle is an acute
triangle.