1/5+1/45+1/117+1/221+1/357简算

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1/5+1/45+1/117+1/221+1/357简算1/5+1/45+1/117+1/221+1/357简算1/5+1/45+1/117+1/221+1/357简算原式=1/(1x5）+1/(5x

1/5+1/45+1/117+1/221+1/357简算
1/5+1/45+1/117+1/221+1/357简算

1/5+1/45+1/117+1/221+1/357简算
原式=1/(1x5）+1/(5x9)+1/(9x13)+1/(13x17)+1/(17x21)
=1/4x(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)
=1/4x20/21
=5/21