Question: Equation with trignometry

Dear All,

I am trying to solve the equation shown in the attached file but can't get the required answer, any suggestions?

I shall be very thankful.




A(n,q) = I(n)*Z(q)/2 * Sum((cos(nwt - qptheta - (n-q)(i-1)2*Pi/3)+cos(nwt + qptheta - (n+q)(i-1)2*Pi/3)),i=1..3)

With n - q = ...-9,-6,-3,0,3,6,9   above equation becomes

A(n,q) = 3/2 *I(n)*Z(q)*cos(nwt-qptheta)  , but I am not getting this result 






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