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①____ Na₂S₂O₃+____HCl→____ NaCl+____S+____SO₂↑+

____H₂O

②____ Zn+____H⁺+____NO₃^﹣→____ Zn²⁺+____NH₄⁺+____H₂O．

表示两个比相等的式子叫做比例。