At that time, the teacher told us to remember this.

There is a formula

commit a crime

α+sin

β=2sin[(α+β)/2] cos[(α-β)/2]

Positive plus positive, go straight.

commit a crime

α-sin

β=2cos[(α+β)/2] sin[(α-β)/2]

Positive and negative, more than before.

cosine

α+cos

β=2cos[(α+β)/2] cos[(α-β)/2]

I add more, I shoulder to shoulder.

cosine

α-cos

β=-2sin[(α+β)/2] sin[(α-β)/2]

I am not as good as me, but I am not.

Explanation: It refers to sine and cosine.

Note that there are coefficients on the right side of this formula. The front row is half the sum of the two angles, and the back row is half the sum of the two angles.

I used to despise memorizing formulas, but later I found it very useful. I am a sophomore now. How many years?

I still remember.

As for the product sum and difference formula, according to this deduction, it comes out.