Explain the Pythagorean theorem.

A. The Pythagorean theorem states that if the square of one side of a triangle is less than the sum of the squares of the other two sides, then the triangle is a right triangle.
B. The Pythagorean theorem states that if the square of one side of a triangle is greater than the sum of the squares of the other two sides, then the triangle is a right triangle.
C. The Pythagorean theorem states that if a triangle is a right triangle, then the sum of the squares of its legs is greater than the square of its hypotenuse.
D. The Pythagorean theorem states that if a triangle is a right triangle, then the sum of the squares of its legs equals the square of its hypotenuse.

D. The Pythagorean theorem states that if a triangle is a right triangle, then the sum of the squares of its legs equals the square of its hypotenuse. This can be expressed as a^2 + b^2 = c^2, where a and b are the lengths of the two shorter sides (legs) of the right triangle, and c is the length of the hypotenuse. The theorem is named after the ancient Greek mathematician Pythagoras.