Trigonometric ratios of general angles

We found that the trigonometric ratios of angles in the four quadrants are related in a certain way. For example, sin 30º = sin 150º, but cos 30º = –cos150º. Why are some ratios negative?

In this video, you will learn to express trigonometric ratios of angles in the 2nd, 3rd and 4th quadrants as ratios of acute angles. This technique is important when you want to solve trigonometric equations. It will also tie up when you learn the graphs of trigonometric functions.