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如果a/b>c/d>0 則每項都乘以bd得到 ad bc 0 若bd為正數則ad>bc 所以隨便取兩個正數如b=2 d=4即可滿足ad>bc 因為a/b>c/d>0 所以a c必須是正數假設取a為3 c為1 即可滿足上面的條件
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