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Sum of cosines cos C + cos D equals which product?
A2 cos((C+D)/2) cos((C−D)/2)
B2 sin((C+D)/2) sin((C−D)/2)
C2 cos((C+D)/2) sin((C−D)/2)
D2 sin((C+D)/2) cos((C−D)/2)
Answer & Solution
Correct answer: A. 2 cos((C+D)/2) cos((C−D)/2)
Two cosines add to twice the cosine of the mean angle times the cosine of half the difference.
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