13. Find the value of
$$
\frac{2007^{2}+2008^{2}-1993^{2}-1992^{2}}{4}
$$
13. Ans: 30000 .
Using $a^{2}-b^{2}=(a+b)(a-b), 2008^{2}-1993^{2}=4001 \times 15$ and $2007^{2}-1992^{2}=3999 \times 15$.
The result follows.