[Index]

	Introduction
	Essence
	Memorizing How to memorize Basic Formulas Sum and Difference Formulas Double & Triple Angle Formulas Half Angle Formulas Product to Sum Formulas Sum to Product Formulas All Formulas
	Understanding Page 1 Page 2 Page 3 Page 4 Page 5
	Summarizing Page 1 Page 2
	Examples Examples Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 General ideas
	Exercises
	Final thoughts

From (1)		(3)
From (2)		(4)

When we look at it, we are very happy, because we already got , and at the same time, we have c.

You might want to square the both sides of those equations. Yes, we will do it, but it's not the right time yet, because we know that we want to have , so we will use the Sum to Product formulas. To do that, we add equations (3) and (4).

(3) + (4)

Dividing 2 from both sides, we have:

Now we are almost there. Next we will just square both sides and we have:

(5)

Again, using our compare strategy, we believe that must be equal to 1, so that the identity will be true. We claim that:

(6)