How to find the sum $\cos(2x)+\cos(4x)+\cos(6x)+\cos(8x)+\ldots+\cos(2016x)$?

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Find the sum $$M= \cos(2x)+\cos(4x)+\cos(6x)+\cos(8x)+\ldots+\cos(2016x)$$

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Invoke the geometric series theorem on this.

$$\sum_{k=1}^{1008} \cos(2kx) = \Re\left(\sum_{k=1}^{1008} e^{2ikx}\right).$$