Posted by Michael on Sunday, March 21, 2010 at 9:14pm.
sometimes it is easier to think in degrees, and then switch back
7π/12 = 105 degrees and 105 = 60+45
both angles that we know
cos 105
= cos(60+45)
= cos60cos45 - sin60sin45
= (1/2)(√2/2) - (√3/2)(√2/2)
= (√2 - √6)/4
so cos7π/12 = (√2-√6)/4
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