Which sum or difference identity would you use to verify that cos (180°–θ) = –cosθ?
A) sin(a-b)=sin a cos b-cos a sin b
B) cos(a-b)=cos a cos b+sin a sin b
C) sin(a-b)=sin a cos b-cos a sin b
D) cos(a+b)=cos a cos b=sin a sin b
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Verified answer
cos(a - b) = cos(a)cos(b) + sin(a)sin(b)
cos(180º - x) = cos(180º)cos(x) + sin(180º)sin(x) = -cos(x)