因为2/12×14=1/84而1/12-1/14=1/84
所以原式:=1/12-1/14+1/14-1/16+1/16-1/18+1/18-1/20+1/20
=1/12
2/12×14+2/14×16+2/16×18+2/18×20+1/20
=(1/12-1/14)+(1/14-1/16)+……+(1/18-1/20)+1/20
=1/12.
2/12×14+2/14×16+2/16×18+2/18×20+1/20
=2*1/2*(1/12-1/14+1/14-1/16+1/16-1/18+1/18-1/20)+1/20
=1/12-1/20+1/20
=1/12
=1/12-1/14+1/14-1/16+1/16-1/18+1/18-1/20+1/20
=1/12
1/12