Write the expression as the cosine of an angle.

Question

cos(11pi/12)cos(7pi/12)+sin(11pi/12)sin(7pi/12)

Answer 1

Very easy! This is in the form:

COS(a)COS(b) + SIN(a)SIN(b) = COS(a - b)

All you have to do is identify which angle is "a" and which is "b" in your example, then plug them in and subtract. They already have an LCD, so this is especially friendly..

Answer 2

cos(A − B) = cos A cos B + sin A sin B  so

 

cos(11pi/12 - 7pi/12) = cos (4pi/12) = cos (pi/3)