Problem We have the following inequality. Show that it has only one integer solution for each $n$. $$k^2+k-2\le2n\le k^2+3k-2$$
Attempt Solving this inequality, I got $$\frac{1}{2}\left(\sqrt{8n+17}-3\right)\leq k\leq\frac{1}{2}\left(\sqrt{8n+9}-1\right)$$ Taking the difference doesn't work since the difference is less than one. What should I do to show that $k$ can only take one integer value.