87 Given $\sin \theta+\cos \theta=\frac{1}{5}$, and $\theta \in(0, \pi)$. Then $\tan \theta=$ $\qquad$
$87-\frac{4}{3}$. Because
$$
\left\{\begin{array}{l}
\sin \theta+\cos \theta=\frac{1}{5}, \\
\sin ^{2} \theta+\cos ^{2} \theta=1,
\end{array}\right.
$$

and $\theta \in(0, \pi)$, so
$$
\sin \theta=\frac{4}{5}, \cos \theta=-\frac{3}{5}, \tan \theta=-\frac{4}{3} .
$$