\(A=\left(\frac{1}{\sqrt{a}+2}+\frac{1}{\sqrt{a}-2}\right):\frac{\sqrt{a}}{a-4}\)\(ĐKXĐ:x\ge0;x\ne4\)

\(A=\frac{\sqrt{a}-2+\sqrt{a}+2}{\left(\sqrt{a}+2\right).\left(\sqrt{a}-2\right)}.\frac{a-4}{\sqrt{a}}\)

\(A=\frac{2\sqrt{a}}{a-4}.\frac{a-4}{\sqrt{a}}\)

\(A=2\)

\(B=\left(\frac{4x}{\sqrt{x}-1}-\frac{\sqrt{x}-2}{x-3\sqrt{x}+2}\right).\frac{\sqrt{x}+1}{x^2}\) \(ĐKXĐ:x\ne1,x>0\)

\(B=\left(\frac{4x.\left(\sqrt{x}-2\right)-\sqrt{x}+2}{\left(\sqrt{x}-1\right).\left(\sqrt{x}-2\right)}\right).\frac{\sqrt{x}-1}{x^2}\)

\(B=\frac{\left(\sqrt{x}-2\right).\left(4x-1\right)}{\left(\sqrt{x}-2\right)}.\frac{1}{x^2}\)

\(B=\frac{4x-1}{1}.\frac{1}{x^2}\)

\(B=\frac{4x-1}{x^2}\)

Vậy.....

Đặt căn thức là A

Ta có: \(\sqrt{2}A=\sqrt{13-2\sqrt{12}}+\sqrt{13+2\sqrt{12}}+2\sqrt{12}\)

\(\Rightarrow\sqrt{2}A=\sqrt{\left(\sqrt{12-1}\right)^2}+\sqrt{\left(\sqrt{12+1}\right)^2}+2\sqrt{12}\)

\(\Rightarrow\sqrt{2}A=\sqrt{12}-1+\sqrt{12}+1+2\sqrt{12}\)

\(\Rightarrow\sqrt{2}A=4\sqrt{2}\)

\(\Rightarrow A=4\sqrt{6}\)

\(A=\sqrt{\left(\frac{\sqrt{x}}{2}\right)^2-2.\frac{\sqrt{x}}{2}.\frac{1}{6}+\frac{1}{36}+\frac{35}{36}}\)

\(=\sqrt{\left(\frac{\sqrt{x}}{2}-\frac{1}{6}\right)^2+\frac{35}{36}\ge}\sqrt{\frac{35}{36}}\)

Dấu "=" khi x = 1/9

Ta có: \(1=abc\le\left(\frac{a+b+c}{3}\right)^3\)

\(\Rightarrow a+b+c\ge3\).

\(\frac{a^2}{b+c}+\frac{b^2}{a+c}+\frac{c^2}{a+b}\ge\frac{\left(a+b+c\right)^2}{2\left(a+b+c\right)}=\frac{a+b+c}{2}\ge\frac{3}{2}\)

Dấu \(=\)khi \(a=b=c=1\).

Bài 7. 

a) \(\sqrt{x+9}=3\)(ĐK: \(x\ge-9\))

\(\Leftrightarrow x+9=3^2\)

\(\Leftrightarrow x=0\)(thỏa mãn) 

b) \(\sqrt{2x^2+2}=3x-1\)

\(\Rightarrow2x^2+2=\left(3x-1\right)^2\)

\(\Leftrightarrow7x^2-6x-1=0\)

\(\Leftrightarrow\orbr{\begin{cases}x=1\\x=-\frac{1}{7}\end{cases}}\)

Thử lại chỉ có \(x=1\)thỏa mãn.

c) \(\sqrt{x^2-2x+1}=19x-1\)

\(\Rightarrow x^2-2x+1=\left(19x-1\right)^2\)

\(\Leftrightarrow360x^2-36x=0\)

\(\Leftrightarrow\orbr{\begin{cases}x=0\\x=\frac{1}{10}\end{cases}}\)

Thử lại chỉ có \(x=\frac{1}{10}\)thỏa mãn. 

d) \(\sqrt{x^2-x-6}=\sqrt{x-3}\)(ĐK: \(x\ge3\)) 

\(\Rightarrow x^2-x-6=x-3\)

\(\Leftrightarrow x^2-2x-3=0\)

\(\Leftrightarrow\orbr{\begin{cases}x=-1\left(l\right)\\x=3\left(tm\right)\end{cases}}\)

e) \(\sqrt{4x^2+4x+1}=\sqrt{x^2+12x+36}\)

\(\Leftrightarrow\left|2x+1\right|=\left|x+6\right|\)

\(\Leftrightarrow\orbr{\begin{cases}2x+1=x+6\\2x+1=-x-6\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}x=5\\x=-\frac{7}{3}\end{cases}}\)

g) \(\sqrt{x+4\sqrt{x-4}}=2\)(ĐK: \(x\ge4\))

\(\Leftrightarrow\sqrt{x-4+4\sqrt{x-4}+4}=2\)

\(\Leftrightarrow\left|\sqrt{x-4}-2\right|=2\)

\(\Leftrightarrow\orbr{\begin{cases}\sqrt{x-4}-2=2\\\sqrt{x-4}-2=-2\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}x=20\\x=4\end{cases}}\)(thỏa mãn) 

Bài 10. 

b) \(\sqrt{x^2-2x+4}=x-1\)

\(\Rightarrow x^2-2x+4=\left(x-1\right)^2\)

\(\Leftrightarrow0x=3\)(vô nghiệm) 

c) \(\sqrt{x^2-6x+9}=4-x\)

\(\Leftrightarrow\left|x-3\right|=4-x\)

\(\Rightarrow\orbr{\begin{cases}x-3=4-x\\x-3=x-4\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}2x=7\\0x=1\end{cases}}\)

\(\Leftrightarrow x=\frac{7}{2}\)(thử lại thỏa mãn) 

d) \(\sqrt{x^2-2x+1}+\sqrt{x^2-4x+4}=3\)

\(\Leftrightarrow\left|x-1\right|+\left|x-2\right|=3\)

Với \(x\ge2\): 

\(x-1+x-2=3\)

\(\Leftrightarrow x=3\)(thỏa mãn) 

Với \(1\le x< 2\):

\(x-1+2-x=3\)

\(\Leftrightarrow0x=2\)(vô nghiệm) 

Với \(x< 1\):

\(1-x+2-x=3\)

\(\Leftrightarrow x=0\)(thỏa mãn) 

\(A=\frac{x+2}{x\sqrt{x}-1}+\frac{\sqrt{x}+1}{x+\sqrt{x}+1}-\frac{1}{\sqrt{x}-1}\)(ĐK: \(x\ge0,x\ne1\)) 

\(=\frac{x+2}{\left(\sqrt{x}-1\right)\left(x+\sqrt{x}+1\right)}+\frac{\left(\sqrt{x}+1\right)\left(\sqrt{x}-1\right)}{\left(\sqrt{x}-1\right)\left(x+\sqrt{x}+1\right)}-\frac{x+\sqrt{x}+1}{\left(\sqrt{x}-1\right)\left(x+\sqrt{x}+1\right)}\)

\(=\frac{x+2+x-1-x-\sqrt{x}-1}{\left(\sqrt{x}-1\right)\left(x+\sqrt{x}+1\right)}\)

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

\(A=\frac{5}{\sqrt{x}}\)

\(\Leftrightarrow\frac{\sqrt{x}}{x+\sqrt{x}+1}=\frac{5}{\sqrt{x}}\)

\(\Rightarrow x=5\left(x+\sqrt{x}+1\right)\)

\(\Leftrightarrow4x+5\sqrt{x}+1=0\)(vô nghiệm do \(x\ge0\)) 

\(A-\frac{1}{3}=\frac{\sqrt{x}}{x+\sqrt{x}+1}-\frac{1}{3}=\frac{3\sqrt{x}-x-\sqrt{x}-1}{3\left(x+\sqrt{x}+1\right)}\)

\(=\frac{-x+2\sqrt{x}-1}{3\left(x+\sqrt{x}+1\right)}=\frac{-\left(\sqrt{x}-1\right)^2}{3\left(x+\sqrt{x}+1\right)}< 0\)(vì \(x\ne1\))

Do đó \(A< \frac{1}{3}\).