Pythagoras’ theorem is always true for right-angled triangles. 130 is not the same as 121. As 𝒂² + 𝒃² does not equal 𝒄², the triangle cannot be a right-angled triangle.

The theorem’s history, however, is more complex than this legend would suggest. The use of the 3-4-5 triangle for constructing a right angle, for instance, goes back to much earlier times in ...

Pythagoras' theorem can also be used to solve 3D problems. You often need to use Pythagoras' theorem more than once in each problem as we create extra right-angled triangles in the 3D shapes.