How do you express $cos( (5 pi)/4 ) * cos (( pi) /3 )$ without using products of trigonometric functions?

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Answer 1 · 2016-02-11T07:14:18

Use cos(A+B) + cos(A-B) = 2 cos(A) cos(B).
The answer is $(cos(19pi/12) + cos(11pi/12))/2$

Here, $A = 5pi/4 and B = pi/3$

How do you express cos( (5 pi)/4 ) * cos (( pi) /3 ) cos( (5 pi)/4 ) * cos (( pi) /3 ) without using products of trigonometric functions?