a ) \(ĐKXĐ:\hept{\begin{cases}x\ge1\\y\ge2\\z\ge3\end{cases}}\)

b) Ta có:

 \(P=\frac{\sqrt{x-1}}{x}+\frac{\sqrt{y-2}}{y}+\frac{\sqrt{z-3}}{z}=\frac{\sqrt{x-1}}{x}+\frac{\sqrt{2}\sqrt{y-2}}{\sqrt{2}y}+\frac{\sqrt{3}\sqrt{z-3}}{\sqrt{3}z}\)

Áp dụng bbđt AM - GM ta có :

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

\(\frac{\sqrt{2}\sqrt{y-2}}{\sqrt{2}y}\le\frac{\frac{2+y-2}{2}}{\sqrt{2}y}=\frac{y}{2\sqrt{2}y}=\frac{1}{2\sqrt{2}}\)

\(\frac{\sqrt{3}\sqrt{z-3}}{\sqrt{3}z}\le\frac{\frac{3+z-3}{2}}{\sqrt{3}z}=\frac{z}{2\sqrt{3}z}=\frac{1}{2\sqrt{3}}\)

\(\Rightarrow P\le\frac{1}{2}+\frac{1}{2\sqrt{2}}+\frac{1}{2\sqrt{3}}\)

Dấu "=" xảy ra khi \(\hept{\begin{cases}x-1=1\\y-2=2\\z-3=3\end{cases}\Rightarrow\hept{\begin{cases}x=2\\y=4\\z=6\end{cases}}}\)

\(x^2-5x+36=8\sqrt{3x+4}\)

\(\Leftrightarrow x^2-5x+36-8\sqrt{3x+4}=0\)

\(\Leftrightarrow\left(-8\sqrt{3x+4}+32\right)+\left(x^2-5x+4\right)=0\)

\(\Leftrightarrow-8\left(\sqrt{3x+4}-4\right)+\left(x-1\right)\left(x-4\right)=0\)

\(\Leftrightarrow-8.\frac{3x+4-16}{\sqrt{3x+4}+4}+\left(x-1\right)\left(x-4\right)=0\)

\(\Leftrightarrow-8.\frac{3x-12}{\sqrt{3x+4}+4}+\left(x-1\right)\left(x-4\right)=0\)

\(\left(x-4\right)\left(\frac{-24}{\sqrt{3x+4}+4}+x-1\right)=0\)

\(\Leftrightarrow\orbr{\begin{cases}x=4\\\frac{-24}{\sqrt{3x+4}+4}+x-1=0\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}x=4\\-\frac{24}{\sqrt{3x+4}+4}+3+x-4=0\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}x=4\\-3.\frac{16-3x-4}{\left(\sqrt{3x+4}+4\right)^2}+\left(x-4\right)=0\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}x=4\\\left(x-4\right)\left[\frac{9}{\left(\sqrt{3x+4}+4\right)^2}+1\right]=0\end{cases}}\)

Mà \(\frac{9}{\left(\sqrt{3x+4}+4\right)^2}+1>0\forall x\) nên \(x-4=0\Rightarrow x=4\)

Vật PT có nghiệm duy nhất là \(x=4\)

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