2.1x3/5和4.5x5/9、3.4x5/6用递等式计算

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2.1*3/5=2.1*6/悄培10=12.6/10=1.26;
4.5*5/9=9*5/9/衫绝2=5/2=2.5;
3.4*5/6=17/6=12/或运姿6+5/6=2+5/6;

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