1/1X4+1/4X7+1/7X10+.+1/20 - 知识问答

1/1X4+1/4X7+1/7X10+.+1/2002X2005=?

1个回答

1/1X4 = （1/1 - 1/4）/3
1/4X7 = (1/4 - 1/7)/3
...
1/2002X2005 = (1/2002 - 1/2005)/3
1/1X4+1/4X7+1/7X10+.+1/2002X2005
=(1/1-1/4 + 1/4 - 1/7 + ... + 1/2002 - 1/2005)/3
=(1-1/2005)/3
=668/2005

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