\(c,=\sqrt{\dfrac{\sqrt{a}-1}{\sqrt{b}-1}:\dfrac{\sqrt{b}-1}{\sqrt{a}-1}}=\sqrt{\dfrac{\left(\sqrt{a}-1\right)^2}{\left(\sqrt{b}-1\right)^2}}=\dfrac{\left|\sqrt{a}-1\right|}{\left|\sqrt{b}-1\right|}=\dfrac{\sqrt{a}-1}{\sqrt{b}-1}\)
\(d,ĐK:xy>0;x\ne y\\ =\dfrac{\sqrt{xy}\left(\sqrt{x}+\sqrt{y}\right)}{\sqrt{xy}}\cdot\left(\sqrt{x}-\sqrt{y}\right)\\ =\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}\right)\\ =x-y\)
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