Answer
$\frac{3}{\pi}.$
Work Step by Step
The average is given by
$$
\frac{1}{\pi / 6-0} \int_{0}^{\pi / 6} \cos x d x=\frac{6}{\pi} \int_{0}^{\pi / 6} \cos x d x=\left.\frac{6}{\pi} \sin x\right|_{0} ^{\pi / 6}=\frac{3}{\pi}.
$$
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