11. In an isosceles right $\triangle A B C$, it is known that $\angle A B C=90^{\circ}$, and the coordinates of points $A, B$ are $A(1,0), B(3,1)$, then the coordinates of vertex $C$ are $\qquad$ .
11. $(2,3)$ or $(4,-1)$ Hint: Let $C(x, y)$, then we have the system of equations: $(3-$
$$
\begin{array}{l}
1)^{2}+(1-0)^{2}=(x-3)^{2}+(y-1)^{2},(x-1)^{2}+(y-0)^{2}= \\
2\left[(3-1)^{2}+(1-0)^{2}\right] .
\end{array}
$$