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Mathematics 9 Online
OpenStudy (anonymous):

find the exact value of cos(15) and cos(7,5)

OpenStudy (jamesj):

You know an exact value for cos(30) ... it's \( \sqrt{3}/2 \). Now, you should also know that cos(2x) = 2cos^2(x) - 1. Can you see how now to calculate cos(15)?

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