小学数学题:10/(1*2)+20/(1*2*3)+30/(1*2*3*4)+·····+90/(1*2*3*···*10)=
来源:百度知道 编辑:UC知道 时间:2024/06/11 11:51:13
没写错
分析:3/1*2*3*4=4/1*2*3*4-1/1*2*3*4=1/1*2*3-1/1*2*3*4
10/(1*2)+20/(1*2*3)+30/(1*2*3*4)+·····+90/(1*2*3*···*10)
=10〔1/(1*2)+2/(1*2*3)+3/(1*2*3*4)+·····+9/(1*2*3*···*10)〕
=10〔1-1/(1*2)+1/(1*2)-1/(1*2*3)+1/(1*2*3)-1/(1*2*3*4)+·····+1/(1*2*3*···*9)-1/(1*2*3*···*10)〕
=10〔1-1/(1*2*3*···*10)〕
=10-1/(1*2*3*···*9)
=10-1/362880
=9又362879/362880
36287/36288
9又362879/362880
能约分的话自己约
10/(1*2)+20/(1*2*3)+30/(1*2*3*4)+·····+90/(1*2*3*···*10)
=10〔1/(1*2)+2/(1*2*3)+3/(1*2*3*4)+·····+9/(1*2*3*···*10)〕
=10〔1-1/(1*2)+1/(1*2)-1/(1*2*3)+1/(1*2*3)-1/(1*2*3*4)+·····+1/(1*2*3*···*9)-1/(1*2*3*···*10)〕
=10〔1-1/(1*2*3*···*10)〕
=10-1/(1*2*3*···*9)
=10-1/362880
=9又362879/362880