680. $y=\log _{5}\left(x^{3}-1\right)$, i.e. $y=\log _{5} u$, where $u=x^{3}-1$.
Solution. $y^{\prime}=\frac{u^{\prime}}{u \ln 5}$ or $y^{\prime}=\frac{\left(x^{3}-1\right)^{\prime}}{\left(x^{3}-1\right) \ln 5}=\frac{3 x^{2}}{\left(x^{3}-1\right) \ln 5}$ [(5); II]. 681. $y=\cos 5^{x}$, i.e., $y=\cos u$, where $u=5^{x}$.

Solution. $y^{\prime}=-\sin u \cdot u^{\prime}$ or $y^{\prime}=-\sin 5^{x}\left(5^{x}\right)^{\prime}=$ $=-5^{x} \sin 5^{x} \ln 5[(7);(4) \S 2]$.