HINT:
Let $\displaystyle\lim_{n\to\infty}\frac{F_{n+1}}{F_n}=u$
Clearly, $u\
ot<0$
By definition, we have $\displaystyle F_{n+1}=F_n+F_{n-1}$
$\displaystyle\implies \frac{F_{n+1}}{F_n}=1+\frac1{\frac{F_n}{F_{n-1}}}$
Setting $\displaystyle n\to\infty, u=1+\frac1u$