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# Need some help on this trig question watch

1. Hey all, i have this question and i cant exactly prove it so i was hoping some1 could help me out.

I have to prove that but i only get as far as

which seems abit wrong to me.

Thanks!

2. You shouldn't have any cos' because they cancel out
3. It doesn't.

Counterexample: A = pi/2, B = 0.

Then sin(pi/2 + 0) + sin(pi/2 - 0) = 2 but not 2sin(pi/2)sin(0) = 0.

I believe you should consider cos(A + B) and cos(A - B).
4. Hmm. That is wrong. .
5. thanks alot
6. (Original post by Swayum)
It doesn't.

Counterexample: A = pi/2, B = 0.

Then sin(pi/2 + 0) + sin(pi/2 - 0) = 2 but not 2sin(pi/2)sin(0) = 0.

I believe you should consider cos(A + B) and cos(A - B).
I meant cosasinb cancels the other one
7. (Original post by boy)
I meant cosasinb cancels the other one
I wasn't replying specifically to you, but rather the OP. By "it doesn't", I meant sin(A + B) + sin(A - B) doesn't equal 2sin(A)cos(B) for all values.
8. (Original post by Swayum)
I wasn't replying specifically to you, but rather the OP. By "it doesn't", I meant sin(A + B) + sin(A - B) doesn't equal 2sin(A)cos(B) for all values.
Oh

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