1/1X3+1/3X5+1/5X7...1/17X19+1/19X21=

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答案和解析

1/1x3+1/3x5+1/5x7+……+1/17x19+1/19x21
=1/2 *(1-1/3+1/3-1/5+.+1/17-1/19+1/19-1/21)
=1/2*(1-1/21)
=10/21

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