\(\sqrt{\dfrac{2a^2b^4}{50}}=\sqrt{\dfrac{a^2b^4}{25}}=\dfrac{b^2\left|a\right|}{5}\)

\(\dfrac{\sqrt{2ab^2}}{\sqrt{162}}=\sqrt{\dfrac{ab^2}{81}}=\dfrac{\sqrt{a}\left|b\right|}{9}\)

a) \(\sqrt{\frac{2a^2b^4}{50}}=\sqrt{\frac{a^2b^4}{25}}=\frac{\sqrt{a^2b^4}}{\sqrt{25}}=\frac{ab^2}{5}\)

b) \(\frac{\sqrt{2ab^2}}{\sqrt{162}}=\sqrt{\frac{2ab^2}{162}}=\sqrt{\frac{ab^2}{81}}=\frac{\sqrt{ab^2}}{\sqrt{81}}=\frac{b\sqrt{a}}{9}\)

B xem lại đề bài thử nhé

bài này mình cũng dò lại đề rồi mình chép đúng đấy mà không làm được nên mới nhờ giải

mờ quá bạn

:v

Đặt A=\(\frac{a-b}{b^2}\sqrt{\frac{a^2b^4}{a^2-2ab+b^2}}=\frac{a-b}{b^2}\sqrt{\frac{a^2b^4}{\left(a-b\right)^2}}=\frac{a-b}{b^2}.\left|\frac{ab^2}{a-b}\right|\)

Với a<b thì : A=\(\frac{a-b}{b^2}.\frac{ab^2}{-\left(a-b\right)}=-a\)

Với a>b thì : A=\(\frac{a-b}{b^2}.\frac{ab^2}{a-b}=a\)

\(\frac{a-b}{b^2}\sqrt{\frac{a^2b^4}{a^2-2ab+b^2}}\)

\(=\frac{a-b}{b^2}\sqrt{\frac{\left(ab^2\right)^2}{\left(a-b\right)^2}}\)

\(=\frac{a-b}{b^2}\cdot\frac{\sqrt{\left(ab^2\right)^2}}{\sqrt{\left(a-b\right)^2}}\)

\(=\frac{a-b}{b^2}\cdot\frac{\left|a\right|b^2}{\left|a-b\right|}\)

+) Nếu a>b => \(\frac{a-b}{b^2}\cdot\frac{ab^2}{a-b}=a\)

+) Nếu a<b => \(\frac{a-b}{b^2}\cdot\frac{ab^2}{b-a}=-a\)

Ta sẽ chứng minh: \(\left(x+y+z\right)\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\ge9\)với x,y > 0.

Thật vậy: \(x+y+z\ge3\sqrt[3]{xyz}\)(bđt Cô -si)

và \(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\ge3\sqrt[3]{\frac{1}{abc}}\)(bđt Cô -si)

\(\Rightarrow\left(x+y+z\right)\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\ge9\)(Dấu "="\(\Leftrightarrow x=y=z\))

Ta có: \(5a^2+2ab+2b^2=\left(2a+b\right)^2+\left(a-b\right)^2\ge\left(2a+b\right)^2\)

\(\Rightarrow\frac{1}{\sqrt{5a^2+2ab+2b^2}}\le\frac{1}{2a+b}\le\frac{1}{9}\left(\frac{1}{a}+\frac{1}{a}+\frac{1}{b}\right)\)

(Dấu "=" xảy ra khi a = b)

Tương tự ta có:\(\frac{1}{\sqrt{5b^2+2bc+2c^2}}\le\frac{1}{2b+c}\le\frac{1}{9}\left(\frac{1}{b}+\frac{1}{b}+\frac{1}{c}\right)\)(Dấu "=" xảy ra khi b=c)

\(\frac{1}{\sqrt{5c^2+2ca+2a^2}}\le\frac{1}{2c+a}\le\frac{1}{9}\left(\frac{1}{c}+\frac{1}{c}+\frac{1}{a}\right)\)(Dấu "=" xảy ra khi c=a)

\(VT=\text{Σ}_{cyc}\frac{1}{\sqrt{5a^2+2ab+b^2}}\le\frac{1}{9}\left(\frac{3}{a}+\frac{3}{b}+\frac{3}{c}\right)\)

\(\le\frac{1}{3}\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\le\frac{2}{3}\)

(Dấu "=" xảy ra khi \(a=b=c=\frac{3}{2}\))

Ô, thanh you, bạn 2k7 sao mà giỏi thế