Special right triangle is 90°, leaving the remaining angle to be 30°. Right triangles whose sides are of integer lengths, with the sides collectively known as Pythagorean triples, possess angles that cannot all be rational numbers of degrees. (This follows from Niven's theorem.) They are most useful in that they may be easily remembered and any multiple of the sides produces the same relationship. Using Euclid's formula for generating Pythagorean triples, the sides must be in the ratio where "m" and "n" are any positive integers such that . There are several Pythagorean triples which are well-known, including those with sides in the
Perceived visual angle Ono & Mapp (2006) noted, it unfortunately has been ignored in many current theories of size perception, and size illusions. The object's other perceived endpoint, A′, has a perceived direction "d′";, about which the observer might say "it appears toward a higher elevation than point B′." The difference between the two perceived directions () is the perceived visual angle "θ′", also called the perceived angular size or apparent angular size. It is not easy to quantify "θ′". For instance, a well-trained observer might say that point A′ "looks about 25 degrees higher" than B′, but most cannot reliably say how