When you buy a geometry set, you are likely to get a protractor as well as two "set squares" which are actually shaped as right-angled triangles.

These triangles have angles 45°, 45°, 90° and 30°, 60°, 90°.

Looking at these triangles more closely, the ratio of their sides are shown in the diagram below.

From this information, we can read off the following trigonometric values using SOHCAHTOA without using a calculator: