Let $f:\mathbb{R}\to(0,\infty)$ be a strictly increasing function such that $\displaystyle \lim_{x\to\infty}\frac{f(7x)}{f(x)}=1$. Then the value of $\displaystyle \lim_{x\to\infty}\Big[\frac{f(5x)}{f(x)}-1\Big]$ is: