1+2+3+4+······+78+79+80

1+2+3+4+······+78+79+80
=（1+80）*（80/2）
=81*40
=3240
1/2+1/6+1/12+1/20+······+1/72+1/90
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+.-1/90
=1-1/90
=89/90
100-99+98-97+96-······+4-3+2-1
=1*50=50
1000-1-2-3-4-······-78-79-80
=1000-（1+2+3+4+5+6+.+80）
=1000-（1+80）*40
=1000-3240
=-2240
1/15+2/15+······+13/15+14/15
=（1/15+14/15 ）*7.5
=7.5