This theorem, which applies to oblique (non-right) triangles, is a generalization of the Pythagorean Theorem.[br][br][i]Try dragging the vertices of the triangle below.[/i]

Trigonometry was not used widely in Edo Japan (see Unger); [i]wasan[/i] practitioners would have known a version of the theorem that avoids reference to angle measure. Note, however, that a change in sign is necessary for obtuse triangles.