1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=?这是道数学题.

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=?这是道数学题.
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=?这是道数学题.

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=?这是道数学题.
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/9-1/10)
=1-1/10
=9/10

等于9/10
希望能帮到你。

对
原式=1-1/2+1/2-1/3+1/3-1/4……+1/9-1/10
=1-1/10
=9/10

=1/（1*2）+1/（2*3）+········1/（9*10）
=（1-1/2）+（1/2-1/3）+········（1/9-1/10）
括号去掉
=1-1/10
=9/10

=1/2+（1/2-1/3）+（1/3-1/4）+（1/4-1/5）+（1/5-1/6）+（1/6-1/7）+（1/7-1/8）+（1/8-1/9）+（1/9-1/10）=1-1/10 =9/10

1/(1*2)+1/(2*3) +1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)+1/(7*8)+1/(8*9)+1/(9*10)
=1-1/2+1/2-1/3+1/3-1/4+....+1/8-1/9+1/9-1/10
=1-1/10
=9/10

=1-1/2+1/2-1/3+...-1/10=0.9

=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+。。。。+1/9-1/10
=1-1/10
=9/10