Here we give you a suggestion: use the three differences to classify all formulas. Memorize which formulas can help us to change angles, and know how the angles are going to change. Which formula can help us to change operations and functions? If you've finished the classification, we think you are good enough to solve almost any problems.

But we also know you need to get used to using our idea to solve problems. So we added Summarizing the formulas section, which will help you understand deeper about these formulas. If you don't feel comfortable to use our idea to solve problems, don't worry, we know it will take a while to learn something new, and we also know you will finally catch our idea.