Answer
$\lim\limits_{x\to\pi}\cos{3x}=-1.$
Work Step by Step
Using both Theorems $1.6$ and $1.5: \lim\limits_{x\to c}(f(g(x)))=f(\lim\limits_{x\to c}g(x)):$
$\lim\limits_{x\to\pi}\cos{3x}=\cos{(\lim\limits_{x\to\pi}3x)}=\cos{3\pi}=-1.$
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