11. Find the value of
$$
\frac{2011^{2}+2111^{2}-2 \times 2011 \times 2111}{25}
$$
11. Answer: 400
$$
\frac{2011^{2}+2111^{2}-2 \times 2011 \times 2111}{25}=\frac{(2011-2111)^{2}}{25}=\frac{100^{2}}{25}=400
$$