4/(1x3)+4/(3x5)+4/(5x7)+...+4/(19x21)
=2/1-2/3+2/3-2/5+2/5-2/7+...+2/19-2/21
=2-2/21
=40/21
4/(1x3)+4/(3x5)+4/(5x7)+...+4/(19x20)=多少?
1个回答
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