\(11-2\sqrt{30}=\left(\sqrt{6}-\sqrt{5}\right)^2\)
\(7-2\sqrt{10}=\left(\sqrt{5}-\sqrt{2}\right)^2\)
\(8+4\sqrt{3}=\left(\sqrt{6}+\sqrt{2}\right)^2\)
Khi đó: \(A=\frac{1}{\sqrt{6}-\sqrt{5}}-\frac{3}{\sqrt{5}-\sqrt{2}}-\frac{4}{\sqrt{6}+\sqrt{2}}\)
\(=\sqrt{6}+\sqrt{5}-\sqrt{5}-\sqrt{2}-\sqrt{6}+\sqrt{2}=0\)