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4/1x3-8/3x5+12/5x7-16/7x9...-40/19x20
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解:原式=(3+1)/(3×1)-(5+3)/(5×3)+(7+5)×(7×5)-……-(19+21)/(19×21)
=1+1/3-1/3-1/5+1/5+1/7-1/7-1/9+...-1/19-1/21
=1+0+...-1/21
=20/21
=1+1/3-1/3-1/5+1/5+1/7-1/7-1/9+...-1/19-1/21
=1+0+...-1/21
=20/21
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