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11pi/12 = pi - pi/12

cos(11pi/12) = cos(pi - pi/12)

cos(a-b) = cos(a)cos(b)+sin(a)sin(b)

cos(pi -pi/12) = cos(pi)cos(pi/12) + sin(pi)sin(pi/12)

sin(pi)=0

cos(pi)=-1

Therefore, cos(pi -pi/12) = -cos(pi/12)

pi/12=pi/3 -pi/4

cos(pi/12) = cos(pi/3 - pi/4)

= cos(pi/3)cos(pi/4)+sin(pi/3) sin(pi/4)

cos(pi/3)=1/2

sin(pi/3)=sqrt(3)/2

cos(pi/4)= sqrt(2)/2

sin(pi/4) = sqrt(2)/2

cos(pi/3)cos(pi/4)+sin(pi/3) sin(pi/4)

= (1/2)(sqrt(2)/2 ) + (sqrt(3)/2)( sqrt(2)/2)

= sqrt(2)/4 + sqrt(6) /4

= [sqrt(2)+sqrt(6)] /4

Therefore, cos(pi/12) = (sqrt(2)+sqrt(6))/4

-cos(pi/12) = -(sqrt(2)+sqrt(6))/4

cos(11pi/12) = -(sqrt(2)+sqrt(6))/4

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