b, Đặt \(\sqrt[3]{x}=t\)

Ta có: \(\sqrt[3]{x^2}-8\sqrt[3]{x}=20\)

\(\Leftrightarrow t^2-8t=20\Leftrightarrow t^2-8t-20=0\)

\(\Leftrightarrow\left(t+2\right)\left(t-10\right)=0\)

\(\orbr{\begin{cases}t=-2\\t=10\end{cases}\Leftrightarrow\orbr{\begin{cases}\sqrt[3]{x}=-2\\\sqrt[3]{x}=10\end{cases}\Leftrightarrow}}\orbr{\begin{cases}x=-8\\x=1000\end{cases}}\)

a/ ĐKXĐ: ...

\(\Leftrightarrow x+8+\sqrt{x+8}-\left(x+8\right)=\sqrt{x}+\sqrt{x+3}\)

\(\Leftrightarrow\sqrt{x+8}=\sqrt{x}+\sqrt{x+3}\)

\(\Leftrightarrow x+8=2x+3+2\sqrt{x^2+3x}\)

\(\Leftrightarrow5-x=2\sqrt{x^2+3x}\) (\(x\le5\))

\(\Leftrightarrow x^2-10x+25=4\left(x^2+3x\right)\)

\(\Leftrightarrow...\)

b/ ĐKXĐ: \(2\le x\le5\)

\(\Leftrightarrow2\left(x-2\right)+\sqrt{2\left(x-2\right)}\left(\sqrt{5-x}-\sqrt{3x-3}\right)=0\)

\(\Leftrightarrow\sqrt{2\left(x-2\right)}\left(\sqrt{2x-4}+\sqrt{5-x}-\sqrt{3x-3}\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=2\\\sqrt{2x-4}+\sqrt{5-x}=\sqrt{3x-3}\left(1\right)\end{matrix}\right.\)

\(\left(1\right)\Leftrightarrow x+1+2\sqrt{\left(2x-4\right)\left(5-x\right)}=3x-3\)

\(\Leftrightarrow\sqrt{\left(2x-4\right)\left(5-x\right)}=x-2\)

\(\Leftrightarrow\left(2x-4\right)\left(5-x\right)=\left(x-2\right)^2\)

\(\Leftrightarrow...\)

c/ ĐKXĐ: \(x\le12\)

\(\Leftrightarrow\sqrt[3]{24+x}\sqrt{12-x}-6\sqrt{12-x}+12-x=0\)

\(\Leftrightarrow\sqrt{12-x}\left(\sqrt[3]{24+x}-6+\sqrt{12-x}\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=12\\\sqrt[3]{24+x}+\sqrt{12-x}=6\left(1\right)\end{matrix}\right.\)

Xét (1):

Đặt \(\left\{{}\begin{matrix}\sqrt[3]{24+x}=a\\\sqrt{12-x}=b\ge0\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}a+b=6\\a^3+b^2=36\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}b=6-a\\a^3+b^2=36\end{matrix}\right.\)

\(\Leftrightarrow a^3+\left(6-a\right)^2=36\)

\(\Leftrightarrow a^3+a^2-12a=0\)

\(\Leftrightarrow a\left(a^2+a-12\right)=0\Rightarrow\left[{}\begin{matrix}a=0\\a=3\\a=-4\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}\sqrt[3]{24+x}=0\\\sqrt[3]{24+x}=3\\\sqrt[3]{24+x}=-4\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}24+x=0\\24+x=27\\24+x=-64\end{matrix}\right.\)

d/ ĐKXĐ: \(x\le\frac{3}{2}\) ; \(x\ne\frac{3}{8};x\ne-\frac{13}{24}\)

\(\Leftrightarrow\left(x-1\right)\left(\frac{1}{2\sqrt{3-2x}-3}-\frac{1}{3-2\sqrt[3]{5+3x}}\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=1\\\frac{1}{2\sqrt{3-2x}-3}=\frac{1}{3-2\sqrt[3]{5+3x}}\left(1\right)\end{matrix}\right.\)

\(\left(1\right)\Leftrightarrow2\sqrt{3-2x}-3=3-2\sqrt[3]{5+3x}\)

\(\Leftrightarrow\sqrt[3]{5+3x}+\sqrt{3-2x}=3\)

Đặt \(\left\{{}\begin{matrix}\sqrt[3]{5+3x}=a\\\sqrt{3-2x}=b\ge0\end{matrix}\right.\) ta được:

\(\left\{{}\begin{matrix}a+b=3\\2a^3+3b^2=19\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}b=3-a\\2a^3+3b^2=19\end{matrix}\right.\)

\(\Leftrightarrow2a^3+3\left(3-a\right)^2=19\)

\(\Leftrightarrow2a^3+3a^2-18a+8=0\)

\(\Rightarrow\left[{}\begin{matrix}a=-4\\a=\frac{1}{2}\\a=2\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}\sqrt[3]{5+3x}=-4\\\sqrt[3]{5+3x}=\frac{1}{2}\\\sqrt[3]{5+3x}=2\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}5+3x=-64\\5+3x=\frac{1}{8}\\5+3x=8\end{matrix}\right.\)

\(A=\left(\frac{\sqrt{3}}{x^2+x\sqrt{x}+3}+\frac{3}{x^3-\sqrt{27}}\right)\left(\frac{x}{\sqrt{3}}+\frac{\sqrt{3}}{x}+1\right)\)

\(\Leftrightarrow A=\left[\frac{\sqrt{3}\left(x-\sqrt{3}\right)}{\left(x-\sqrt{3}\right)\left(x^2+x\sqrt{3}+3\right)}+\frac{3}{\left(x-\sqrt{3}\right)\left(x+x\sqrt{3}+3\right)}\right]\left(\frac{x^2+3+x\sqrt{3}}{x\sqrt{3}}\right)\)

\(\Leftrightarrow A=\frac{x\sqrt{3}-3+3}{\left(x-\sqrt{3}\right)\left(x^2+x\sqrt{3}+3\right)}.\frac{x^2+x\sqrt{3}+3}{x\sqrt{3}}\)

\(\Leftrightarrow A=\frac{1}{x-\sqrt{3}}\)

em ms hok lớp 1