The Pythagorean Theorem
In a right triangle, the legs squared add to the hypotenuse squared.
Concept
Only for right triangles
Label the two sides that meet at the right angle a and b, and the long side opposite the right angle c. Then a² + b² = c².
A few triples show up constantly — 3-4-5, 5-12-13, 8-15-17 — and their multiples. Recognizing them saves you the square roots.
a² + b² = c²