The Pythagorean Theorem is a highly useful formula in mathematics. The relationship between the three side lengths of a right triangle is so important, that many cultures discovered its use simultaneously.

Pyramids at Giza

The Egyptians used the Pythagorean Theorem to measure heights and distances.

The Pythagorean Theorem uses squares to relate the side lengths of a right triangle. The sum of the areas of the squares whose sides make up the legs of the right triangle is equal to the area of the square whose side makes up the hypotenuse of the right triangle.

Three squares forming a right triangle when joined at their vertices

In other words,
right triangle with two legs and hypotenuse labeled

Sources of images used for this section as they appear, top to bottom:

Ikiwaner, All Gizah Pyramids, Wikimedia Commons