276. $u=\log _{5} \cos 7 x$.

276. $u=\log _{5} \cos 7 x$.
Solution. $u^{\prime}=\frac{1}{\cos 7 x \ln 5}(\cos 7 x)^{\prime}=\frac{-\sin 7 x \cdot 7}{\cos 7 x \ln 5}=-\frac{7 \operatorname{tg} 7 x}{\ln 5}$.