The quadrant rule or CAST diagram is very useful to know as it saves you having to draw graphs to find a set of solutions to an equation in a given range. I would strongly recommend that you learn this method then you can go on to use it to solve trigonometric equations as seen in the 2nd video below.

Quadrant Rule or CAST diagram : ExamSolutions Maths Revision Video Tutorials - youtube Video

Using the quadrant rule to solve simple trigonometric equations

Using the Quadrant Rule to solve trig. equations | ExamSolutions Maths Tutorials - youtube Video