2/12*14+2/14*16+2/16*18+2/18*20+2/20*22+2/20*22+1/22
=(14/12*14-12/12*14)+（16/14*16-14/14*16）.+1/12
=1/12-1/14+1/14-1/16+.-1/22+1/22
=1/12