Watch the video and then solve the problem given below Use a

Watch the video and then solve the problem given below. Use a sum or difference identity to find the exact value of the expression. cos 5 pi/12 cos 5 pi/12 = (Simplify your answer, including any radicals. Use integers or fractions for

Solution

cos(5/12) = cos75^o

Using the sum identity:

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

cos 75 = cos(45 + 30) = cos45*cos30 – sin45*sin30

           = (2/2)* (3/2) - (2/2)*(1/2) = (6/4) - (2/4) = (6 - 2)/4