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What is the exact value of the expression cos 7pi over 12 cos pi over 6 -sin 7pi over 12 sin pi over 6?
Wiki User
∙ 12y ago
cos(a)cos(b)-sin(a)sin(b)=cos(a+b)
a=7pi/12 and b=pi/6
a+b = 7pi/12 + pi/6 = 7pi/12 + 2pi/12 = 9pi/12
We want to find cos(9pi/12)
cos(9pi/12) = cos(3pi/4)
cos(3pi/4)= cos(pi-pi/4)
cos(pi)cos(pi/4)-sin(pi)sin(pi/4)
cos(pi)=-1
sin(pi)=0
cos(pi/4) = √2/2
sin(pi/4) =√2/2
cos(pi)cos(pi/4)-sin(pi)sin(pi/4) = - cos(pi/4) = -√2/2
Wiki User
∙ 12y ago
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