薄荷红茶1985
幼苗
共回答了1099个问题 举报
1/3+1/6+1/10+1/15+1/21+1/28+1/36+1/45
=(1/3+1/6)+(1/10+1/15)+(1/21+1/28)+(1/36+1/45)
=1/3(1+1/2)+1/5(1/2+1/3)+1/7(1/3+1/4)+1/9(1/4+1/5)
=1/3*3/2+1/5*5/6+1/7*7/12+1/9*9/20
=1/2+1/6+1/12+1/20
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)
=1-1/5
=4/5
1年前
2