Question

15. In AABC, the value of cos (A - B) - cos (C) is:

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Answer 1

Answer:

2cosAcosB

Step-by-step explanation:

A+B+C=180.... triangle sum law

C=180-(A+B)

substituting c in question

cos(A-B)-cos[180-(A+B)]

now cos(180-theta)= -cos theta

cos(A-B)+cos(A+B)

applying cosC+cosD= 2cos(C+D)/2 cos(C-D)/2

2cosAcos(-B)....as cos(-theta)=cos theta

2cosAcosB

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