11. If $a, b \in \mathbf{R}$ and $a^{2}+b^{2}=10$, then the range of values for $a-b$ is $\qquad$
11. $[-2 \sqrt{5}, 2 \sqrt{5}]$.

Given $a, b \in \mathbf{R}$ and $a^{2}+b^{2}=10$, we have
$$
\begin{array}{l}
(a-b)^{2}=2\left(a^{2}+b^{2}\right)-(a+b)^{2} \\
\leqslant 2\left(a^{2}+b^{2}\right)=20 .
\end{array}
$$

Thus, $|a-b| \leqslant 2 \sqrt{5}$.