列项求和
1/(1*2)=1-1/2
1/(2*3)=1/2-1/3
......
1/(9*10)=1/9-1/10
原式=1-1/2+1/2-1/3+1/3-1/4+。。。+1/9-1/10
=1-1/10
=9/10
原式=1-1/2+1/2-1/3+1/3......+1/9-1/10
=1-1/10
=9/10
1/1*2+1/2*3+1/3*4+……+1/9*10
=(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+……+(1/9-1/10)
=1/1-1/2+1/2-1/3+1/3-1/4+……+1/9-1/10
=1/1-1/10
=9/10
1/1*2+1/2*3+1/2*3+1/3*4…+1/9*10
=1-1/2+1/2-1/3+1/3-1/4+……+1/9-1/10
=1-1/10
=9/10