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# help with this math question!?

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- JohnathanLv 71 month ago
cos(a - b) = cos(a)cos(b) + sin(a)sin(b). Remember this.

The first two answers are already wrong because they're looking for sine in both cases: answer 1 is sin(120° + 45°) or sin(165°), answer 2 is sin(120° - 45°), or sin(75°).

Answer 4 doesn't work because this uses an angle sum: that's cos(120° + 45°), or cos(165°).

The correct usage of the cosine of an angle difference is answer 3. (There's a typo in both this and answer 4; the second cos(45°) should be sin(45°), though both of those are the same value.)

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