+ \(\sqrt{20+\sqrt{20}}< \sqrt{20+\sqrt{25}}\)
\(\Rightarrow\sqrt{20+\sqrt{20}}< \sqrt{25}\)
\(\Rightarrow\sqrt{20+\sqrt{20}}< 5\)
\(\Rightarrow\sqrt{20+\sqrt{20+\sqrt{20}}}< \sqrt{20+5}\)
\(\Rightarrow\sqrt{20+\sqrt{20+\sqrt{20}}}< 5\)
Tương tự như vậy ta có :
\(A=\sqrt{20+\sqrt{20+\sqrt{20+...+\sqrt{20}}}}< \sqrt{20+5}\)
\(\Rightarrow A< 5\)