# Thread: Trig. Establishing Identity

1. ## Trig. Establishing Identity

My problem is:

Establish the identity.

cos (α + β) + cos (α - β) = 2 cos α cos β

Any help would be greatly appreciated. Thanks!

2. Originally Posted by xhazel06x My problem is:

Establish the identity.

cos (α + β) + cos (α - β) = 2 cos α cos β

Any help would be greatly appreciated. Thanks!
The trig identity is the Sum/Difference Identity:

$\displaystyle cos(\alpha ^{+}_{-} \beta) = cos\alpha cos\beta ^{+}_{-} sin\alpha sin\beta$

Can you take it from here?

3. I understand that it's a sum and difference identity. I assume it's best to do the right side? Or is it best to start on the right? So confuse! I'm not very good with trig. identities what so ever.

4. Originally Posted by xhazel06x I understand that it's a sum and difference identity. I assume it's best to do the right side? Or is it best to start on the right? So confuse! I'm not very good with trig. identities what so ever.

Leave the right side alone. Take the left side and evaluate it using the sum/difference formula and I'll bet you will end up with what is on the right side. All the question is asking you to do is establish the identity or prove that what is one the left is equal to what is on the right. You can do that by just evaluating the left side.

5. Originally Posted by mollymcf2009 Leave the right side alone. Take the left side and evaluate it using the sum/difference formula and I'll bet you will end up with what is on the right side. All the question is asking you to do is establish the identity or prove that what is one the left is equal to what is on the right. You can do that by just evaluating the left side.
Oh, wow that was simple! Thanks!

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