Posted by Michael on Sunday, March 21, 2010 at 9:14pm.
Find the exact value of cos7(pi)/12 using sum and/or difference identities.

Trigonometric  Reiny, Sunday, March 21, 2010 at 11:02pm
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 
Trigonometric  Yomar, Sunday, April 12, 2015 at 4:27pm
cos 7π/12