2*{1/(2*5)}+4*{1/(5*8)}+6*{1/(8*11)}+.......+100*{1/(149*152)}+102*{1/(152*155)}=?
来源:百度知道 编辑:UC知道 时间:2024/09/26 04:24:47
2*{1/(2*5)}+4*{1/(5*8)}+6*{1/(8*11)}+.......+100*{1/(149*152)}+102*{1/(152*155)}=?
给出步骤,谢谢
给出步骤,谢谢
设n1=1/(2×5)=1/3(1/2-1/5)
n2=1/(5×8)=1/3(1/5-1/8)
……
n51=1/(152×155)=1/3(1/152-1/155)
则,n1+n2+n3+n4+……+n51=1/3(1/2-1/155)
n2+n3+n4+……+n51=1/3(1/5-1/155)
n3+n4+……+n51=1/3(1/8-1/155)
……
n50+n51=1/3(1/149-1/155)
n51=1/3(1/152-1/155)
原式=2×(n1+n2+n3+n4+……+n51)+2×(n2+n3+n4+……+n51)+2×(n3+n4+……+n51)+……+2×(n50+n51)+2×n51
=(2/3)×[(1/2-1/155)+(1/5-1/155)+(1/8-1/155)+……+(1/152-1/155)]
=(2/3)×[(1/2+1/5+1/8+……+1/152)-51/155]
只能求到这一步了,小括号内的1/2+1/5+1/8+……+1/152既非等差数列亦非等比数列,不能用现成公式精确求出。
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