5+10+15+20+25+...+85+90+95+100
=（5+100）/2*100/5
=1050
(1+3+5+7+...+1991)-(2+4+6+8+...+1990)
=1-2+3-4+……+1989-1990
=（-1）*1990/2
=-995
3.7=37/10