初一数学题(在线等,急急急!!!!)
1/19×21 +1/21×23 +1/23×25 +...+ 1/97×99=??
1/19×21 +1/21×23 +1/23×25 +...+ 1/97×99 =1/2[(1/19-1/21)+(1/21-1/23)+(1/23-1/25)+....(1/97-1/99)] =1/2(1/19-1/99) =(99-19)/(2*19*99) =40/(19*99) =40/1881 =0.021...
=1/2(1/19-1/21)+1/2(1/21-1/23)+...+1/2(1/97-1/99) =1/2(1/19-1/21+1/21-1/23+1/23-1/25+...+1/97-1/99) =1/2(1/19-1/99) =40/1881
原式 =0.5*(1/19-1/21+1/21-1/23+1/23-1/25+…+1/97-1/99) =0.5*(1/19-1/99) =40/1881
答:1/19×21 +1/21×23 +1/23×25 +...+ 1/97×99 =1/2[(1/19-1/21)+(1/21-1/23)+(1/23-1/25)...详情>>
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