Answer
$1$
Work Step by Step
We expand and simplify the sum:
$\displaystyle \sum_{k=0}^{8}\cos k\pi=\cos 0\pi+\cos1\pi+\cos 2\pi+\cos 3\pi+\cos 4\pi+\cos 5\pi+\cos 6\pi+\cos 7\pi+\cos 8\pi$
$=1-1+1-1+1-1+1-1+1$
$=1$
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