`sum_(x=1)^n cos(x alpha)`

`sum_(x=1)^n cos(x alpha) = (sin(( "n" +1/2) alpha ))/(2sin( alpha /2)) - 1/2`

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This is the sum of cosines of multiples of the angle `alpha`: `cos(alpha)+ cos(2alpha)+...+cos(n alpha)` Note: if n is not an integer, the fractional portion of the real is truncated to create an integer.

Reference

This formula is also found in Schaum's Mathematical Handbook (19.39)