1又1/12+2又1/20+3又1/30+4又1/42=1+1/12+2+1/20+3+1/30+4+1/42=(1+2+3+4)+(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7)=10+(1/3-1/7)=10+4/21=10又4/21
