9. In the $n$-th order Farey sequence, let $a / b, a^{\prime} / b^{\prime}$ be the fractions adjacent to $1 / 2$ on the left and right, respectively. Prove: $b=b^{\prime}=1+2[(n-1) / 2]$, i.e., $b$ is the largest odd number not exceeding $n$, and $a+a^{\prime}=b$.