17. $y=\frac{x^{2}-x+n}{x^{2}+x+1}$ ( $n$ is a positive integer) has a minimum value of $a_{n}$, a maximum value of $b_{n}$, and $c_{n}=\frac{n}{4}\left(1+3 a_{n} b_{n}\right)$. Prove:
$$\frac{3}{2}-\frac{1}{n+1}<\sum_{k=1}^{n} \frac{1}{c_{k}}<2-\frac{1}{n}(n \geqslant 2)$$