\(\text{a, Ta có:}\)
\(3\sqrt{7}=\sqrt{3^27}=\sqrt{63}\)
\(9=\sqrt{81}\)
\(\text{Vì}:\sqrt{81}>\sqrt{63}\Rightarrow3\sqrt{7}< 9\)
\(\text{b, Vì}\) \(-\sqrt{3}>-\sqrt{5}\Rightarrow-\sqrt{\sqrt{3}}>-\sqrt{\sqrt{5}}\)
\(c,\sqrt{51}-\sqrt{3}\approx5,4>5\)
\(d.\text{Vì}\) \(5>\sqrt{5}\Rightarrow\sqrt{85+5}>\sqrt{85+\sqrt{5}}\)