Calcea Johnson: So to start the proof, we start with just a regular right triangle where the angle ... So then what we do next is we draw a second congruent, which means they're equal in size.

Triangles ABC and GHI are congruent. Triangle GHI is the same as ABC but has been rotated 90° anticlockwise. Triangle DEF is a different size. Image caption, Line segments AB and GH are the same ...