9. 5 cos (π + arccos 0,1).

1) $$arccos(0.1) = a$$

2) $$cos(\pi + a) = -cos(a)$$

3) $$cos(arccos(0.1)) = 0.1$$

4) $$-cos(a) = -0.1$$

5) $$5 \cdot (-0.1) = -0.5$$

Ответ: -0.5