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= \(\sqrt{12-2.2\sqrt{3}.5+25}-\sqrt{12+2.2\sqrt{3}.5+25}\)
= \(\sqrt{\left(2\sqrt{3}-5\right)^2}-\sqrt{\left(2\sqrt{3}+5\right)^2}\)
= \(|2\sqrt{3}-5|-2\sqrt{3}-5\)
=\(5-2\sqrt{3}-2\sqrt{3}-5=-4\sqrt{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\dfrac{1}{2-\sqrt{3}}+\dfrac{1}{2+\sqrt{3}}-\sqrt{37-20\sqrt{3}}\)
\(=\dfrac{2+\sqrt{3}}{\left(2-\sqrt{3}\right)\left(2+\sqrt{3}\right)}+\dfrac{2-\sqrt{3}}{\left(2-\sqrt{3}\right)\left(2+\sqrt{3}\right)}-\sqrt{\left(5-2\sqrt{3}\right)^2}\)
\(=2+\sqrt{3}+2-\sqrt{3}-5+2\sqrt{3}\)
\(=2\sqrt{3}-1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
`a)(\sqrt{14}-3\sqrt{2})^2+6\sqrt{28}`
`=14-12\sqrt{7}+18+12\sqrt{7}=32`
`b)2\sqrt{20}-3\sqrt{20}+\sqrt{125}`
`=4\sqrt{5}-6\sqrt{5}+5\sqrt{5}`
`=3\sqrt{5}`.
a) \(\left(\sqrt{14}-3\sqrt{2}\right)^2-6\sqrt{28}\)
\(=\left(\sqrt{14}\right)^2-2\cdot\sqrt{14}\cdot3\sqrt{2}+\left(3\sqrt{2}\right)^2+6\sqrt{28}\)
\(=14-6\sqrt{28}+18+6\sqrt{28}\)
\(=14+18\)
\(=32\)
b) \(2\sqrt{20}-3\sqrt{20}+\sqrt{125}\)
\(=2\cdot2\sqrt{5}-3\cdot2\sqrt{5}+5\sqrt{5}\)
\(=4\sqrt{5}-6\sqrt{5}+5\sqrt{5}\)
\(=3\sqrt{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài 1 :
Ta có :
\(\sqrt{37-20\sqrt{3}}+\sqrt{37+20\sqrt{3}}=\sqrt{25-2.5.2\sqrt{3}+12}\)
\(+\sqrt{25+2.5.2\sqrt{3}+12}\)
\(=\sqrt{\left(5-2\sqrt{3}\right)^2}+\sqrt{\left(5+2\sqrt{3}\right)^2}\)
\(5-2\sqrt{3}+5+2\sqrt{3}\)
\(=5+5=10\)
Bài 2 :
Với x , y , z > 0 . Ta có :
+ ) \(\frac{x}{y}+\frac{y}{x}\ge2\left(1\right)\)
+ ) \(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\ge\frac{9}{x+y+z}\left(2\right)\)
+ ) \(x^2+y^2+z^2\ge xy+yz+zx\Leftrightarrow\frac{x^2+y^2+z^2}{xy+yz+zx}\ge1\left(3\right)\)
Xảy ra đăng thức ở : \(\left(1\right),\left(2\right),\left(3\right)\Leftrightarrow x=y=z\) . Ta có :
\(P=\frac{ab+bc+ca}{a^2+b^2+c^2}+\left(a+b+c\right)^2.\frac{\left(a+b+c\right)}{abc}\)
\(=\frac{ab+bc+ca}{a^2+b^2+c^2}+\left(a^2+b^2+c^2+2ab+2bc+2ca\right).\frac{\left(a+b+c\right)}{abc}\)
Áp dụng các bất đẳng thức (1) , (2) , (3) ta được :
\(P\ge\frac{ab+bc+ca}{a^2+b^2+c^2}+\left(a^2+b^2+c^2\right).\frac{9}{ab+bc+ca}+2.9\)
\(=\left(\frac{ab+bc+ca}{a^2+b^2+c^2}+\frac{a^2+b^2+c^2}{ab+bc+ca}\right)+8.\frac{a^2+b^2+c^2}{ab+bc+ca}+18\)
\(\ge2+8+18=28\)
Dấu " = " xảy ra \(\Leftrightarrow\hept{\begin{cases}a^2+b^2+c^2=ab+bc+ca\\ab=bc=ca\end{cases}\Leftrightarrow a=b=c}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) Ta có: \(A^3=\left(\sqrt[3]{2+\sqrt{5}}+\sqrt[3]{2-\sqrt{5}}\right)^3\)
\(=2+\sqrt{5}+2-\sqrt{5}+3\cdot\sqrt[3]{\left(2+\sqrt{5}\right)\left(2-\sqrt{5}\right)}\left(\sqrt[3]{2+\sqrt{5}}+\sqrt[3]{2-\sqrt{5}}\right)\)
\(=4-3\cdot A\)
\(\Leftrightarrow A^3+3A-4=0\)
\(\Leftrightarrow A^3-A+4A-4=0\)
\(\Leftrightarrow A\left(A-1\right)\left(A+1\right)+4\left(A-1\right)=0\)
\(\Leftrightarrow\left(A-1\right)\left(A^2+A+4\right)=0\)
\(\Leftrightarrow A=1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1.
Ta có: \(E=\sqrt{37-6\sqrt{30}}=\sqrt{\left(3\sqrt{3}-\sqrt{10}\right)^2}=\left|3\sqrt{3}-\sqrt{10}\right|=3\sqrt{3}-\sqrt{10}\)
\(F=\sqrt{51-6\sqrt{30}}=\sqrt{\left(3\sqrt{5}-\sqrt{6}\right)^2}=\left|3\sqrt{5}-\sqrt{6}\right|=3\sqrt{5}-\sqrt{6}\)
\(G=\sqrt{59-6\sqrt{30}}=\sqrt{\left(3\sqrt{6}-\sqrt{5}\right)^2}=\left|3\sqrt{6}-\sqrt{5}\right|=3\sqrt{6}-\sqrt{5}\)
\(H=\sqrt{17-2\sqrt{30}}=\sqrt{\left(\sqrt{15}-\sqrt{2}\right)^2}=\left|\sqrt{15}-\sqrt{2}\right|=\sqrt{15}-\sqrt{2}\)
\(E=\sqrt{37-6\sqrt{30}}\\ =\sqrt{\left(3\sqrt{3}-\sqrt{10}\right)^2}\\ =\left|3\sqrt{3}-\sqrt{10}\right|\\ =3\sqrt{3}-\sqrt{10}\)
\(F=\sqrt{51-6\sqrt{30}}\\ =\sqrt{\left(3\sqrt{5}-\sqrt{6}\right)^2}\\ =\left|3\sqrt{5}-\sqrt{6}\right|\\ =3\sqrt{5}-\sqrt{6}\)
\(G=\sqrt{59-6\sqrt{30}}\\ =\sqrt{\left(3\sqrt{6}-\sqrt{5}\right)^2}\\ =\left|3\sqrt{6}-\sqrt{5}\right|\\ =3\sqrt{6}-\sqrt{5}\)
\(H=\sqrt{17-2\sqrt{30}}\\ =\sqrt{\left(\sqrt{15}-\sqrt{2}\right)^2}\\ =\left|\sqrt{15}-\sqrt{2}\right|=\sqrt{15}-\sqrt{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a: \(=2\sqrt{20\sqrt{3}}-2\sqrt{5\sqrt{3}}-3\cdot\sqrt{20\sqrt{3}}\)
\(=4\sqrt{5\sqrt{3}}-2\sqrt{5\sqrt{3}}-6\sqrt{5\sqrt{3}}=-4\sqrt{5\sqrt{3}}\)
b: \(=2\sqrt{5\sqrt{3}}-4\sqrt{2\sqrt{3}}-6\sqrt{5\sqrt{3}}=-4\sqrt{5\sqrt{3}}-4\sqrt{2\sqrt{3}}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a: Ta có: \(2\sqrt{28}+2\sqrt{63}-3\sqrt{175}+\sqrt{112}-\sqrt{20}\)
\(=4\sqrt{7}+6\sqrt{7}-15\sqrt{7}+4\sqrt{7}-2\sqrt{5}\)
\(=-\sqrt{7}-2\sqrt{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\sqrt{20}-\sqrt{45}+3\sqrt{18}+\sqrt{72}\)
\(=\sqrt{4.5}-\sqrt{9.5}+3\sqrt{18}+\sqrt{4.18}\)
\(=2\sqrt{5}-3\sqrt{5}+3\sqrt{18}+2\sqrt{18}\)
\(=-\sqrt{5}+5\sqrt{18}\)
\(=\sqrt{\left(5-2\sqrt{3}\right)^2}+\sqrt{\left(5+2\sqrt{3}\right)^2}\)
\(=\left|5-2\sqrt{3}\right|+\left|5+2\sqrt{3}\right|\)
\(=5-2\sqrt{3}+5+2\sqrt{3}\)
\(=10\)