Also known as Pythagoras' theorem, the Pythagorean theorem is a theorem in geometry that gives a relationship between the sides of a right-angled triangle. The Pythagorean theorem was evidently known before Pythagoras (6th century B.C.), but the proof in general form is ascribed to him. Originally the theorem established a relationship between the areas of the squares constructed on the sides of a right-angled triangle: The square on the hypotenuse is equal to the sum of the squares on the other sides. Sometimes, the Pythagorean theorem is formulated briefly as follows: The square of the hypotenuse of a right-angled triangle is equal to the sum of the squares of the catheti. The theorem converse to the Pythagorean theorem is also true: If the square of a side of a triangle is equal to the sum of the squares of the other two sides, then that triangle is right-angled.
The problem of solving Pythagoras' equation $a^2+b^2=c^2$ in integers $a,b,c$ leads to the Pythagorean numbers. The problem of solving its generalization, the Diophantine equation $a^n+b^n=c^n$, $n\ge3$, is called Fermat's last (or great) theorem, cf. Fermat great theorem.
A right-angled triangle with sides having integer lengths is called a Pythagorean triangle.
|W.H. Greub, "Linear algebra" , Springer (1967)
Pythagoras theorem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Pythagoras_theorem&oldid=20501