计算 (1/11+1/21+1/31+1/41)*(1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+1/51)*(1/21+1/31+1/41)
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计算 (1/11+1/21+1/31+1/41)*(1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+1/51)*(1/21+1/31+1/41)
计算 (1/11+1/21+1/31+1/41)*(1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+1/51)*(1/21+1/31+1/41)
计算 (1/11+1/21+1/31+1/41)*(1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+1/51)*(1/21+1/31+1/41)
=(1/11+1/21+1/31+1/41)*[(1/21+1/31+1/41+1/51)-(1/21+1/31+1/41)]-(1/51)*(1/21+1/31+1/41)
=(1/11+1/21+1/31+1/41)*(1/51)-(1/51)*(1/21+1/31+1/41)
=(1/51)*[(1/11+1/21+1/31+1/41)-(1/21+1/31+1/41)]
=(1/51)*(1/11)
=1/561
(1/11+1/21+1/31+1/41)*(1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+1/51)*(1/21+1/31+1/41)
=(1/11+1/21+1/31+1/41)*[(1/21+1/31+1/41+1/51)-(1/21+1/31+1/41)]-(1/51)*(1/21+1/31+1/41)
=(1/11+1/21+1/31+1/41)*(1/51)-(1/51)*(1/21+1/31+1/41)
=(1/51)*[(1/11+1/21+1/31+1/41)-(1/21+1/31+1/41)]
=(1/51)*(1/11)
=1/561
等于1/11*1/51
（1/11+1/21+1/31+1/41）*（1/21+1/31+1/41+1/51）-（1/11+1/21+1/31+1/41+/51）*(1/21+1/31+1/41）
=【1/11+（1/21+1/31+1/41）】*（1/21+1/31+1/41+1/51）-【1/11+（1/21+1/31+1/41+/51）】*(1/21+1/31+1/41）
=1/11*（1/21+1...
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（1/11+1/21+1/31+1/41）*（1/21+1/31+1/41+1/51）-（1/11+1/21+1/31+1/41+/51）*(1/21+1/31+1/41）
=【1/11+（1/21+1/31+1/41）】*（1/21+1/31+1/41+1/51）-【1/11+（1/21+1/31+1/41+/51）】*(1/21+1/31+1/41）
=1/11*（1/21+1/31+1/41+1/51）+（1/21+1/31+1/41）（1/21+1/31+1/41+1/51）-1/11*（1/21+1/31+1/41）-（1/21+1/31+1/41）*（1/21+1/31+1/41+1/51）
=1/11*（1/51）=1/561
最后一步有无可避免的两位数乘两位数
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