395. Find the values of $\cos x, \operatorname{tg} x, \operatorname{ctg} x$, if it is known that $\sin x=-3 / 5, 0<x<3 \pi / 2$.
Solution. Since the angle $x$ ends in the III quadrant, then $\cos x < 0$ and $\operatorname{ctg} x > 0$.

Using the value of $\sin x$ and the formula $\cos ^{2} x=1-\sin ^{2} x$, we find $\cos ^{2} x=1-\left(-\frac{3}{5}\right)^{2}=1-\frac{9}{25}=\frac{16}{25}$, from which $\cos x=-\frac{4}{5} ; \operatorname{tg} x=\frac{3}{4} ;$ $\operatorname{ctg} x=\frac{4}{3}$.