b: \(B=\dfrac{\sqrt{20}}{\sqrt{5}}+\dfrac{\left(\sqrt{18}+\sqrt{8}\right)}{\sqrt{2}}-\sqrt{2}\cdot\sqrt{8}\)
\(=\sqrt{\dfrac{20}{5}}+\dfrac{3\sqrt{2}+2\sqrt{2}}{\sqrt{2}}-\sqrt{16}\)
\(=\sqrt{4}+3+2-4=2+3+2-4=3\)
d: \(D=\left[\dfrac{1-\sqrt{2}}{1+\sqrt{2}}-\dfrac{1+\sqrt{2}}{1-\sqrt{2}}\right]:\sqrt{72}\)
\(=\dfrac{\left(1-\sqrt{2}\right)\left(1-\sqrt{2}\right)-\left(1+\sqrt{2}\right)\left(1+\sqrt{2}\right)}{\left(1+\sqrt{2}\right)\left(1-\sqrt{2}\right)}:6\sqrt{2}\)
\(=\dfrac{3-2\sqrt{2}-3-2\sqrt{2}}{1-2}:6\sqrt{2}\)
\(=-\dfrac{4\sqrt{2}}{-1}:6\sqrt{2}=\dfrac{4\sqrt{2}}{6\sqrt{2}}=\dfrac{2}{3}\)
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