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Cos (7 pi/ 12)

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3 Answers

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!

cos (7pi/12) = cos (7 · 180o / 12) = cos 105o ≈ - 0.26
             180o / 12 = 15o ---> 15o · 7 = 105o

Try using the half angle formula:

cos (x/2) = +/- √[(1 + cos x)/2]

since 7pi/12 = 1/2 (7 pi/6), 

cos (7pi/12) = +/- √[(1 + cos (7pi/6))/2]

I'll leave the rest to the student.