`sum_(x=0)^n(r^x cos(x alpha))`

`sum_(x=0)^n(r^x cos(x alpha)) = ( "r" ^( "n" +2) cos( "n" alpha ) - "r" ^( "n" +1) cos[( "n" +1) alpha ] - "r" cos( alpha ) +1 ) / (1 - 2 "r" cos( alpha ) + "r" ^2)`

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This is the sum of an progression of integers (`r^n`) times the cosine of a function of `n alpha`: `1 + r cos(alpha) + r^2 cos(2 alpha)+...+ r^n cos(n alpha)`