3. 17 (1) Simplify $\frac{1-a^{2}}{(1+a x)^{2}-(a+x)^{2}}$;
(2) When $x=0.44$, find the value of $\sqrt{1-x-x^{2}+x^{3}}$.
[Solution]
(1)
$$
\begin{aligned}
\text { Original expression } & =\frac{(1+a)(1-a)}{(1+a x+a+x)(1+a x-a-x)} \\
& =\frac{(1+a)(1-a)}{(1+a)(1+x)(1-a)(1-x)} \\
& =\frac{1}{(1+x)(1-x)} .
\end{aligned}
$$
(2)
$$
\begin{aligned}
& \sqrt{1-x-x^{2}+x^{3}}=\sqrt{(1-x)-x^{2}(1-x)} \\
= & \sqrt{(1-x)\left(1-x^{2}\right)}=\sqrt{(1-x)^{2}(1+x)} \\
= & |1-x| \sqrt{1+x}=|1-0.44| \sqrt{1+0.44} \\
= & 0.56 \times 1.2 \\
= & 0.672 .
\end{aligned}
$$