Lời giải:
\(\frac{1}{\sqrt{7}}+\frac{1}{\sqrt{11}}> \frac{1}{\sqrt{4}}+\frac{1}{\sqrt{9}}=\frac{5}{6}>\frac{4}{6}=\frac{2}{3}\)

a/ giả sử \(\sqrt{7}-\sqrt{2}< 1\)

\(\Leftrightarrow\sqrt{7}< 1+\sqrt{2}\)

\(\Leftrightarrow 7< 1+2\sqrt{2}+2\)

\(\Leftrightarrow4< 2\sqrt{2}\Leftrightarrow16< 8\left(sai\right)\)

vậy \(\sqrt{7}-\sqrt{2}>1\)

câu b, c bạn làm tương tụ nhé , giả sử một đẳng thức tạm, sau đó bình phương lên rồi làm theo như trên là được nha 

Bài này cũng dễ

a, \(\sqrt{7}-\sqrt{2}\) lớn hơn \(1\) . Vì

\(\sqrt{7}-\sqrt{2}=1,231537749\)

\(1=1\)

b, \(\sqrt{8}+\sqrt{5}\) bé hơn \(\sqrt{7}+\sqrt{6}\) . Vì

\(\sqrt{8}+\sqrt{5}=5,064495102\) 

\(\sqrt{7}+\sqrt{6}=5,095241054\)

c, \(\sqrt{2005}+\sqrt{2007}\) lớn hơn \(\sqrt{2006}\) . Vì

\(\sqrt{2005}+\sqrt{2007}=89,57677992\)

\(\sqrt{2006}=44,78839135\) 

\(\left\{{}\begin{matrix}a=\dfrac{35}{49}=\dfrac{5}{7}\\b=\sqrt{\dfrac{5^2}{7^2}}=\dfrac{5}{7}\\c=\dfrac{\sqrt{5^2}+\sqrt{35^2}}{\sqrt{7^2}+\sqrt{49^2}}=\dfrac{5+35}{7+49}=\dfrac{5}{7}\\d=\dfrac{\sqrt{5^2}-\sqrt{35^2}}{\sqrt{7^2}-\sqrt{49^2}}=\dfrac{5-35}{7-49}=\dfrac{5}{7}\end{matrix}\right.\)

\(\Rightarrow a=b=c=d=\dfrac{5}{7}\)

\(a=\dfrac{35}{49};b=\dfrac{5}{7}\\ c,=\dfrac{5+35}{7+49}=\dfrac{12}{14}=\dfrac{6}{7}\\ d,=\dfrac{5-35}{7-49}\)

Áp dụng t/c dtsbn:

\(\dfrac{5}{7}=\dfrac{35}{49}=\dfrac{5+35}{7+49}=\dfrac{5-35}{7-49}\) hay \(a=b=c=d\)

a) \(1=\sqrt{1}< \sqrt{2}\)

b) \(2=\sqrt{4}>\sqrt{3}\)

c) \(6=\sqrt{36}< \sqrt{41}\)

d) \(7=\sqrt{49}>\sqrt{47}\)

e) \(2=1+1=\sqrt{1}+1< \sqrt{2}+1\)

f) \(1=2-1=\sqrt{4}-1>\sqrt{3}-1\)

g) \(2\sqrt{31}=\sqrt{4.31}=\sqrt{124}>\sqrt{100}=10\)

h) \(\sqrt{3}>0>-\sqrt{12}\)

i) \(5=\sqrt{25}< \sqrt{29}\)

\(\Rightarrow-5>-\sqrt{29}\)

\(\sqrt{2}B=\sqrt{8-2\sqrt{7}}+2=\sqrt{\left(\sqrt{7}-1\right)^2}+2=\sqrt{7}-1+2=\sqrt{7}+1\)

\(\sqrt{2}A=\sqrt{8+2\sqrt{7}}=\sqrt{\left(\sqrt{7}+1\right)^2}=\sqrt{7}+1\)

Vậy A = B 

A = 11 

B = 7 

--> A > B 

\(A\sqrt{2}=\sqrt{8+2\sqrt{7}}=\sqrt{\left(\sqrt{7}\right)^2+2\sqrt{7}+1}=\sqrt{\left(\sqrt{7}+1\right)^2}=\sqrt{7}+1\)

\(B\sqrt{2}=\sqrt{8-2\sqrt{7}}+\left(\sqrt{2}\right)^2=\sqrt{\left(\sqrt{7}-1\right)^2}+2=\sqrt{7}-1+2=\sqrt{7}+1\)

=> \(A\sqrt{2}=B\sqrt{2}\) => A = B

a) Ta có:

\(2=1+1=1+\sqrt{1}\)

Mà: \(1< 2\Rightarrow\sqrt{1}< \sqrt{2}\)

\(\Rightarrow1+\sqrt{1}< \sqrt{2}+1\)

\(\Rightarrow2< \sqrt{2}+1\)

b) Ta có:

\(1=2-1=\sqrt{4}-1\)

Mà: \(4>3\Rightarrow\sqrt{4}>\sqrt{3}\)

\(\Rightarrow\sqrt{4}-1>\sqrt{3}-1\)

\(\Rightarrow1>\sqrt{3}-1\)

c) Ta có:

\(10=2\cdot5=2\sqrt{25}\)

Mà: \(25< 31\Rightarrow\sqrt{25}< \sqrt{31}\)

\(\Rightarrow2\sqrt{25}< 2\sqrt{31}\)

\(\Rightarrow10< 2\sqrt{31}\)

d) Ta có:

\(-12=-3\cdot4=-3\sqrt{16}\)

Mà: \(16>11\Rightarrow\sqrt{16}>\sqrt{11}\)

\(\Rightarrow-3\sqrt{16}< -3\sqrt{11}\)

\(\Rightarrow-12< -3\sqrt{11}\)

\(A=\sqrt{6+2\sqrt{5}}-\sqrt{5}=\sqrt{5}+1-\sqrt{5}=1\)

\(B=\sqrt[3]{7+5\sqrt{2}}-\sqrt{2}=\sqrt{2}+1-\sqrt{2}=1\)

Do đó: A=B

\(\sqrt{6+2\sqrt{5}}-\sqrt{5}=\sqrt{\left(\sqrt{5}+1\right)^2}-\sqrt{5}=\left|\sqrt{5}+1\right|-\sqrt{5}=1\)

\(\sqrt[3]{7+5\sqrt{2}}-\sqrt{2}=\sqrt[3]{\left(\sqrt{2}\right)^3+1^3+3.2+3\sqrt{2}}-\sqrt{2}=\sqrt[3]{\left(\sqrt{2}+1\right)^3}-\sqrt{2}=\sqrt{2}+1-\sqrt{2}=1\)

--> Bằng nhau

2/ 

a) Ta có:

\(3\sqrt{2}=\sqrt{3^2\cdot2}=\sqrt{9\cdot2}=\sqrt{18}\)

\(2\sqrt{3}=\sqrt{2^2\cdot3}=\sqrt{4\cdot3}=\sqrt{12}\)

Mà: \(12< 18\Rightarrow\sqrt{12}< \sqrt{18}\Rightarrow2\sqrt{3}< 3\sqrt{2}\)

b) Ta có:

\(4\sqrt[3]{5}=\sqrt[3]{4^3\cdot5}=\sqrt[3]{320}\)

\(5\sqrt[3]{4}=\sqrt[3]{5^3\cdot4}=\sqrt[3]{500}\)

Mà: \(320< 500\Rightarrow\sqrt[3]{320}< \sqrt[3]{500}\Rightarrow4\sqrt[3]{5}< 5\sqrt[3]{4}\)

3/

a)ĐKXĐ: \(x\ne1;x\ge0\)

b) \(A=\left(1-\dfrac{x-\sqrt{x}}{\sqrt{x}-1}\right)\left(1+\dfrac{x+\sqrt{x}}{\sqrt{x}+1}\right)\)

\(A=\left[1-\dfrac{\sqrt{x}\left(\sqrt{x}-1\right)}{\sqrt{x}-1}\right]\left[1+\dfrac{\sqrt{x}\left(\sqrt{x}+1\right)}{\sqrt{x}+1}\right]\)

\(A=\left(1-\sqrt{x}\right)\left(1+\sqrt{x}\right)\)

\(A=1^2-\left(\sqrt{x}\right)^2\)

\(A=1-x\)

1/ \(\sqrt[3]{54}-\sqrt[3]{16}\)

\(=\sqrt[3]{3^3\cdot2}-\sqrt[3]{2^3\cdot2}\)

\(=3\sqrt[2]{3}-2\sqrt[3]{2}\)

\(=\left(3-2\right)\sqrt[3]{2}\)

\(=\sqrt[3]{2}\)