Triangle inequality ≥ b ≥ c" and "formula_8" is the golden ratio, as In the case of right triangles, the triangle inequality specializes to the statement that the hypotenuse is greater than either of the two sides, and less than their sum. The second part of this theorem is already established above for any side of any triangle. The first part is established using the lower figure. In the figure, consider the right triangle . An isosceles triangle is constructed with equal sides . From the triangle postulate, the angles in the right triangle satisfy: Likewise, in the isosceles triangle , the
Heronian triangle triangles with these sidelengths, and joins them together along the sides of length "a", to obtain a triangle with integer side lengths "c", "e", and "b" + "d", and with area If "a" is even then the area "A" is an integer. Less obviously, if "a" is odd then "A" is still an integer, as "b" and "d" must both be even, making "b"+"d" even too. Some Heronian triangles cannot be obtained by joining together two right-angled triangles with integer sides as described above. For example, a 5, 29, 30 Heronian triangle with area 72 cannot be constructed from two