3. The value of $x$ when the algebraic expression $4-x^{2}-\sqrt{1-x^{2}}$ reaches its minimum is $\qquad$ ; the value of $x$ when the algebraic expression $4-x^{2}-\sqrt{1-x^{2}}$ reaches its maximum is $\qquad$ .
3. When the minimum value is reached, $x= \pm \frac{\sqrt{3}}{2} ;$ when the maximum value is reached, $x= \pm 1,0$.
Notice that
$$
\begin{array}{l}
4-x^{2}-\sqrt{1-x^{2}}=3+\left(1-x^{2}\right)-\sqrt{1-x^{2}} \\
=\left(\sqrt{1-x^{2}}-\frac{1}{2}\right)^{2}+\frac{11}{4} .
\end{array}
$$

When $\sqrt{1-x^{2}}=\frac{1}{2}$, i.e., $x= \pm \frac{\sqrt{3}}{2}$, the expression takes the minimum value $\frac{11}{4}$; when $\sqrt{1-x^{2}}=0$ or 1, i.e., $x= \pm 1,0$, the expression takes the maximum value 3.