Trigonometry. Help needed.

I would look at using product to sum identities.
 
As suggested above, use:

2 * cos(A) * cos(B) = cos(A+B) + cos(A-B)
 
Thanks, I've figured it out how to solve this. :)
I have another question, does the equation sinx=sin1 has infinitely many solutions (answers) ?
 
Thanks, I've figured it out how to solve this. :)
I have another question, does the equation sinx=sin1 has infinitely many solutions (answers) ?

Yes.

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