\begin{array}{|c|c|c|c|c|c|c|c|}
\hline
 \square  & \square  & 0 & \square  & \frac{b}{a}<1 & \square  & 1 & \square  \\
\hline
 x-1 & - & | & - & | & - & 0 & + \\
\hline
 x-\frac{b}{a} & - & | & - & 0 & + & | & + \\
\hline
 x & - & 0 & + & | & + & | & + \\
\hline
 f(x) & - & | & + & | & - & | & + \\
\hline
\end{array}