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а) \(\cfrac{a+\sqrt{7}}{7-a^{2}}=\) [ ];

б) \(\cfrac{x-225}{\sqrt{x}-15}=\) [ ];

в) \(\cfrac{a-4b}{\sqrt{a}-2\sqrt{b}}=\) [ ];

г) \(\cfrac{5\sqrt{a}-7\sqrt{b}}{25a-49b}=\) [ ].