How do you express $cos (\frac{5 π}{4}) \cdot cos (\frac{π}{3})$ $cos( (5 pi)/4 ) * cos (( pi) /3 )$ 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 $\frac{cos (19 \frac{π}{12}) + cos (11 \frac{π}{12})}{2}$ $(cos(19pi/12) + cos(11pi/12))/2$

Here, $A = 5 \frac{π}{4} and B = \frac{π}{3}$ $A = 5pi/4 and B = pi/3$

How do you express cos(5π4)⋅cos(π3)cos( (5 pi)/4 ) * cos (( pi) /3 ) without using products of trigonometric functions?