Welcome to right-angled trigonometry. This is a powerful tool in mathematics that lets us find missing side lengths or angles in right-angled triangles. We'll learn how to label the sides of a triangle correctly, and then use a famous mnemonic, SOHCAHTOA, to set up and solve problems.

First, let's learn how to name the sides. Every right-angled triangle has three special names for its sides, but their names depend on which angle you are focusing on. However, one side's name is always the same. That's the hypotenuse. It's always the longest side, and you can always find it directly opposite the right angle.

The other two sides are called the opposite and the adjacent. Their names change depending on which angle, other than the right angle, we are interested in. The 'opposite' side is the one directly across from our angle of interest. The 'adjacent' side is the one next to our angle of interest, but it's not the hypotenuse.