Find the value of $\cos 1^\circ + \cos 2^\circ + \cos 3^\circ +.....+\cos 180^\circ $
My Attempt:$$=\cos 1^\circ + \cos 2^\circ + \cos 3^\circ +....+\cos 180^\circ $$$$= \cos 1^\circ + \cos 2^\circ + \cos 3^\circ +....-1$$
How do I complete it?
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