1+2+3+4+······+78+79+801/2+1/6+1/12+1/20+······+1/72+1/90100-99+98-97+96-······+4-3+2-11000-1-2-3-4-······-78-79-801/15+2/15+······+13/15+14/15注：例 1/15 就是 十五分之一

1)1+2+3+4+······+78+79+80=(1+80)+(2+79)+......+(39+42)+(40+41)
=81*40=3240
2)1/2+1/6+1/12+1/20+······+1/72+1/90
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+......(1/8-1/9)+(1/9-1/10)
=1-1/10
=9/10
3)100-99+98-97+96-······+4-3+2-1
=(100-99)+(98-97)+(96-95)+······+(4-3)+(2-1)
=1*50
=50
4)1000-1-2-3-4-······-78-79-80
=1000-(1+2+3+4+······+78+79+80)（第一题结果已算出）
=1000-3240=-2240
5)1/15+2/15+······+13/15+14/15
=1/15*(1+2+3+.......+14)
=1/15*(15*7)(方法同第一题）
=7

1/15+2/15+······+13/15+14/15 =105/15=7
1000-1-2-3-4-······-78-79-80=-2240
1+2+3+4+······+78+79+80=3240

1+2+3+4+······+78+79+80=（1+80）*（80/2）=81*40=32401/2+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=89/90100-99+98-97+96-······+4-3+2-1=1*50=501000-1-2...