You use the trigonometric ratios when you are given a side and an angle in a right angle triangle and are asked to find a missing side.

Note: Which ratio to use will depend on the data you are given about the triangle.

Here are some examples below

If you are asked to find an angle in a right angle trianle and are given two sides then, in this case, you use the INVERSE TRIGONOMETRIC functions.

Circle Theorems (I - III)
Rounding to - the nearest... / a significant figure