17. How many roots has the equation $\sin x-\log _{10} x=0$ ?
(ans. 3.
$\sin x=\log x \Rightarrow x \leq 10.100$, of course). Since $2 \pi+\frac{\pi}{2}=\frac{5 \pi}{2}<10, \sin x$ intersect $\log x$ at two points to the left and right side of $\frac{5 \pi}{2}$.)