11+12-13-14+15+16-17-18+.+99+100
=11+12+（-13-14+15+16)+(-17-18+19+20)+...+(-97-98+99+100)
=11+12+4*(100-12)/4
=11+12+88
=111
-1+3-5+7-9+11-13+.+1995-1997+1999
=2*(1999-1)/2*1/2
=999
1991乘1999-1990乘2000
=(1995^2-4^2)-(1995^2-5^2)
=5^2-4^2
=9