# Book demonstration about trigonometric relations

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1. May 9, 2017

### Bunny-chan

1. The problem statement, all variables and given/known data

In the equation between $(3)$ and $(2)$, why does the author says that ? Isn't the trigonometric identity actually ?

2. Relevant equations

3. The attempt at a solution

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2. May 9, 2017

### FactChecker

You are right, but that doesn't mean that what they say is wrong. In fact it proves that their statements are correct. They want to prove something about the absolute value, so they took the absolute value of both sides.

3. May 10, 2017

### Bunny-chan

Sorry, I don't understand what you mean. How does it prove his statement is correct?

I wasn't wondering about the absolute values, I was confused about the fact that it should be two times the cosine, not the sine.

4. May 10, 2017

### FactChecker

It's the same thing. Multiplication is commutative. They just swapped the order of the multiplication.