Prove that cos2Acos3A−cos2Acos7A+cosAcos10Asin4Asin3A−sin2Asin5A+sin4Asin7A=cot6Acot5A
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Solution
Multiply above and below by 2 and apply LHS (cos5A+cosA)−(cos9A+cos5A)+(cos11A+cos9A)(cosA+cos7A)−(cos3A+cos7A)+(cos3A+cos11A)=(cosA+cos11A)(cosA−cos11A)=2cos6Acos5A2sin6Asin5A=cot6Acot5A