22. Find the minimum value of the function $f(x)=\cos 3 x+4 \cos 2 x+8 \cos x, x \in \mathbf{R}$.
22. $f(x)=4 \cos ^{3} x-3 \cos x+4\left(2 \cos ^{2} x-1\right)+8 \cos x=4 \cos ^{3} x+8 \cos ^{2} x+5 \cos x-4=(\cos x+$
1) $(2 \cos x+1)^{2}-5 \geqslant-5$.
When and only when $\cos x=-1$ or $\cos x=-\frac{1}{2}$, the equality holds.