I'm sorry, but I don't understand what this means...? I mean, yes, you can "turn it into 1" and thus "get" 1, but what was your mathematical reasoning for "turning" something into some value you picked?

A good way to start is to convert everything into sines and cosines:

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How did you go from the above to "1"?

Factor, and then see what you can do with whatever is left:

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I don't know how "simplified" your book wants you to go, so you'll have to decide where to go from the above.

The customary technique is to work on the more-complicated side, converting everything to sines and cosines, usually, and applying whatever identities seem useful.

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. . . . .

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...and so forth.

The left-hand side would appear to be the more-complicated one, so:

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...and so forth.

Again, the left-hand side appears to be more complex, so:

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...and so forth.

What were this instructions on this one? What have you tried? How far have you gotten? Where are you stuck?

In future, it might help if you showed that work, so that we can "see" where you're having trouble and then provide intelligent advice. Thank you!