6. Let $x^{2}+y^{2}=a^{2}, a^{2}<1$. Try to find the maximum and minimum values of $S=\sqrt{1-x^{2}}+$ $\sqrt{1-y^{2}}$.
(Answer;
$$\left.S_{\text {large }}=\sqrt{4-3 a^{2}}, S_{\text {small }}=1+\sqrt{1-a^{2}}\right)$$
)