Question

[Trần Ngân Hà]

Ko sử dụng máy tính,hãy so sánh :

a) 3+\(\sqrt{5}\)Và \(2\sqrt{2}+\sqrt{6}\).                   b)\(\sqrt{27}+\sqrt{26}+1\)và\(\sqrt{48}\)                   c)\(\sqrt{105}-\sqrt{101}\)và \(\sqrt{101}-\sqrt{97}\)                                                       d) \(\sqrt{5\sqrt{3}}\)và \(\sqrt{3\sqrt{5}}\)                              e)\(\frac{1}{\sqrt{1}}+\frac{1}{\sqrt{2}}+...\frac{1}{\sqrt{36}}\) và 14

Answer 1

[Vũ Quang Vinh]

a) Ta thấy:
\(\left(3+\sqrt{5}\right)^2=\left(\sqrt{9}+\sqrt{5}\right)^2=9+5+2\sqrt{45}=14+2\sqrt{45}\)
\(\left(2\sqrt{2}+\sqrt{6}\right)^2=\left(\sqrt{8}+\sqrt{6}\right)^2=8+6+2\sqrt{48}=14+2\sqrt{48}\)
Vì \(45< 48\)
\(\Rightarrow\sqrt{45}< \sqrt{48}\)
\(\Rightarrow2\sqrt{45}< 2\sqrt{48}\)
\(\Rightarrow14+2\sqrt{45}< 14+2\sqrt{48}\)
\(\Rightarrow\left(3+\sqrt{5}\right)^2< \left(2\sqrt{2}+\sqrt{6}\right)^2\)
Do \(3+\sqrt{5}>0;2\sqrt{2}+\sqrt{6}>0\)
\(\Rightarrow3+\sqrt{5}< 2\sqrt{2}+6\)

Answer 2

[Vũ Quang Vinh]

b) Ta thấy:
Vì \(26>3\)
\(\Rightarrow\sqrt{26}>\sqrt{3}\)
\(\Rightarrow\sqrt{26}+1>\sqrt{3}\)
\(\Rightarrow\sqrt{27}+\sqrt{26}+1>\sqrt{27}+\sqrt{3}\)
Mà \(\sqrt{27}+\sqrt{3}=3\sqrt{3}+\sqrt{3}=4\sqrt{3}=\sqrt{48}\)
\(\Rightarrow\sqrt{27}+\sqrt{26}+1>\sqrt{48}\)