2/12*14+2/14*16+2/16*18+2/18*20=1/12-1/14+1/14-1/16+1/16-1/20=1/12-1/20=1/30
原式=7/3+8/3+9/8+1/20 =16+1/8+1/20 =16+7/40
6.175