13. Find all sets of integers $A, B$, and $C$, for which

$$
A^{2}-B^{2}-C^{2}=1, \quad B+C-A=3
$$
80.13. Answer. $A=9, B=8, C=4$ or $A=9, B=4, C=$ $=8$, or $A=-3, B=2, C=-2$, or $A=-3, B=-2, C=$ $=2$.