Question 728125
Find the exact value by using a half angle identity. 
cos 5pi/12
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cos s/2=±√[(1-cos s)/2]
cos(5π/12)=cos(5π/6)/2
=±√[(1+cos(5π/6)/2]
select positive sign because 5π/12 is in quadrant I where cos>0
cos(5π/12)=√[(1-√3/2)/2]

note: (Sign change is a correction to the original solution)