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1x3
=(2-1)(2+1)
=2^2-1
得
2^2=1x3+1
其他同理
原式
=(1x3+1)/(1x3)+(3x5+1)/(3x5)+...+(19x21+1)/19x20
=1+1/1x3+1+1/3x5+...+1+1/19x21
=19+1/1x3+1/3x5+...+1/19x21
=19+1/2x(1-1/3+1/3-1/5+...+1/19-1/21)
=19+1/2x20/21
=19又10/21

稍等哈