Довести тотжність cos5a+cos8a+cos9a+cos12a=4cos3a/2*cos2a*cos 17a/2

Question

Довести тотжність cos5a+cos8a+cos9a+cos12a=4cos3a/2*cos2a*cos 17a/2

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Answer 1

(сos5a+cos9a)+(cos8a+cos12a)=2cos2acos7a+2cos2acos10a=
=2cos2a(cos7a+cos10a)=2cos2a*2cos(3a/2)*cos(17a/2)=
=4cos2a*cos(3a/2)*cos(17a)/2