If two 90 degree angles are placed back-to-back (Figure 5), they total 180 degrees. The sides of this angle form a straight line, so a 180 degree angle is a straight line. Adding two more 90 degree ...

A simple guide on how to show the angles in triangle add up to 180 degrees, aimed at students aged 11 to 14.

I'm talking about trigonometry, of course: that bastion of angles and triangles that is essential to calculating velocity, momentum, and much more.