Find the exact value of each expression

The best way to answer this question is with the sum formula for cosine. cos (a + b) = cos a * cos b - sin a * sin b cos (7pi/12) = cos (3pi/12 + cos 4pi/12) = cos (pi/4 + pi/3) Good luck and let me know if you still need help!

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Sorry for the messy post, I can't seem to get the html formatting to work so it's all run together...