Triangle inequality is an exterior angle of a right triangle and therefore larger than , meaning the other base angle of the isosceles triangle also is greater than and their sum exceeds in violation of the triangle postulate). This theorem establishing inequalities is sharpened by Pythagoras' theorem to the equality that the square of the length of the hypotenuse equals the sum of the squares of the other two sides. Consider a triangle whose sides are in an arithmetic progression and let the sides be , , . Then the triangle inequality requires that The first of these quadratic inequalities requires to
Pythagorean theorem used as a triangle "C" on its hypotenuse, and two similar right triangles ("A" and "B" ) constructed on the other two sides, formed by dividing the central triangle by its altitude. The sum of the areas of the two smaller triangles therefore is that of the third, thus "A" + "B" = "C" and reversing the above logic leads to the Pythagorean theorem a + b = c. ("See also Einstein's proof by dissection without rearrangement") The Pythagorean theorem is a special case of the more general theorem relating the lengths of sides in any triangle, the law of