1. Use interval notation to represent the domain of the function
$$
f(x)=\ln \left(\frac{1-x}{x+3}-1\right)
$$

as . $\qquad$
$-1 .(-3,-1)$.
From $\frac{1-x}{x+3}-1>0$, we get
$$
\frac{-2-2 x}{x+3}>0 \Rightarrow-3<x<-1 \text {. }
$$

Therefore, the domain of $f(x)$ is $(-3,-1)$.