a: \(A=\dfrac{2\sqrt{a}-9}{a-5\sqrt{a}+6}-\dfrac{\sqrt{a}+3}{\sqrt{a}-2}-\dfrac{2\sqrt{a}-1}{3-\sqrt{a}}\)

\(=\dfrac{2\sqrt{a}-9-\left(\sqrt{a}+3\right)\left(\sqrt{a}-3\right)+\left(2\sqrt{a}-1\right)\left(\sqrt{a}-2\right)}{\left(\sqrt{a}-2\right)\left(\sqrt{a}-3\right)}\)

\(=\dfrac{2\sqrt{a}-9-a+9+2a-5\sqrt{a}+2}{\left(\sqrt{a}-2\right)\cdot\left(\sqrt{a}-3\right)}\)

\(=\dfrac{a-3\sqrt{a}+2}{\left(\sqrt{a}-2\right)\left(\sqrt{a}-3\right)}=\dfrac{\sqrt{a}-1}{\sqrt{a}-3}\)

b: A là số nguyên

=>\(\sqrt{a}-3+2⋮\sqrt{a}-3\)

=>\(\sqrt{a}-3\in\left\{1;-1;2;-2\right\}\)

=>a thuộc {16;25;1}

b: ĐKXĐ: \(x\in R\)

c: ĐKXĐ: \(\left[{}\begin{matrix}x\ge1\\x\le0\end{matrix}\right.\)

cmr là cái j

Lê Thanh Thùy Ngân 

cmr là chứng minh rằng bạn nhé 

\(a,P=\left(\dfrac{1}{\sqrt{x}-2}-\dfrac{1}{\sqrt{x}+2}\right)\cdot\left(\dfrac{\sqrt{x}+2}{2}\right)^2\left(x\ge0;x\ne4\right)\\ P=\dfrac{\sqrt{x}+2-\sqrt{x}+2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\cdot\dfrac{\left(\sqrt{x}+2\right)^2}{4}\\ P=\dfrac{4}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\cdot\dfrac{\left(\sqrt{x}+2\right)^2}{4}=\dfrac{\sqrt{x}+2}{\sqrt{x}-2}\)

\(b,\)Ta có \(x=6-2\sqrt{5}=\left(\sqrt{5}-1\right)^2\)

Thay vào \(P\), ta được:

\(P=\dfrac{\sqrt{\left(\sqrt{5}-1\right)^2}+2}{\sqrt{\left(\sqrt{5}-1\right)^2}-2}=\dfrac{\sqrt{5}-1+2}{\sqrt{5}-1-2}=\dfrac{\sqrt{5}+1}{\sqrt{5}-3}\)

\(c,\)Để \(P< 1\Leftrightarrow\dfrac{\sqrt{x}+2}{\sqrt{x}-2}< 1\)

\(\Leftrightarrow\dfrac{\sqrt{x}+2}{\sqrt{x}-2}-1< 0\\ \Leftrightarrow\dfrac{\sqrt{x}+2-\sqrt{x}+2}{\sqrt{x}-2}< 0\\ \Leftrightarrow\dfrac{4}{\sqrt{x}-2}< 0\\ \Leftrightarrow\sqrt{x}-2< 0\left(4>0\right)\\ \Leftrightarrow\sqrt{x}< 2\\ \Leftrightarrow x< 4\)

Vậy để \(P< 1\) thì \(x< 4\)

Tick nha

a: ĐKXĐ: \(\left\{{}\begin{matrix}x\ge0\\x\ne4\end{matrix}\right.\)

Ta có: \(P=\left(\dfrac{1}{\sqrt{x}-2}-\dfrac{1}{\sqrt{x}+2}\right)\cdot\left(\dfrac{\sqrt{x}+2}{2}\right)^2\)

\(=\dfrac{\sqrt{x}+2-\sqrt{x}+2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\cdot\dfrac{\left(\sqrt{x}+2\right)^2}{4}\)

\(=\dfrac{\sqrt{x}+2}{\sqrt{x}-2}\)

b: Thay \(x=6-2\sqrt{5}\) vào P, ta được:

\(P=\dfrac{\sqrt{5}+1+2}{\sqrt{5}+1-2}=\dfrac{3+\sqrt{5}}{\sqrt{5}+1}=\dfrac{1+\sqrt{5}}{2}\)

\(b,\)\(\sqrt{\frac{2}{x^2}}\)

Căn thức xác định \(\Leftrightarrow\frac{2}{x^2}\)thỏa mãn đkxđ

\(\Rightarrow x^2\ne0\)

\(\Rightarrow x\ne0\)

a) \(\sqrt{\frac{-5}{x^2+6}}\)

Để biểu thức có nghĩa thì \(x^2+6< 0\)

Mà \(x^2\ge0\Rightarrow x^2+6\ge6\)(mâu thuẫn)

Vậy biểu thức này không xác định

c) \(\sqrt{\frac{1}{-1+x}}\)

Để biểu thức xác định thì \(-1+x\ge1\)

\(\Leftrightarrow x\ge2\)

Vậy \(ĐKXĐ:x\ge2\)

/ kí hiệu là trên

\(a,\)\(đkxđ\Leftrightarrow x\ge0\)và \(x-9\ne0\Rightarrow x\ne9\)

\(A=\frac{6\sqrt{x}}{x-9}-\frac{5\sqrt{x}}{3-\sqrt{x}}+\frac{\sqrt{x}}{\sqrt{x}+3}\)

\(\)\(=\frac{6\sqrt{x}}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}+\frac{5\sqrt{x}\left(\sqrt{x}+3\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}+\frac{\sqrt{x}\left(\sqrt{x}-3\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\)

\(=\frac{6\sqrt{x}+5x+15\sqrt{x}+x-3\sqrt{x}}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\)

\(=\frac{18\sqrt{x}+6x}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\)

\(=\frac{6\sqrt{x}\left(\sqrt{x}+3\right)}{\left(\sqrt{x}+3\right)\left(\sqrt{x}-3\right)}=\frac{6\sqrt{x}}{\sqrt{x}-3}\)

\(b,\)Để \(A>2\)\(\Rightarrow\frac{6\sqrt{x}}{\sqrt{x}-3}>2\)

\(\Rightarrow\frac{6\sqrt{x}}{\sqrt{x}-3}>\frac{12\sqrt{x}}{x-3}\)

\(\Rightarrow\frac{6\sqrt{x}-12\sqrt{x}}{\sqrt{x}-3}>0\)

\(\Rightarrow\frac{6\sqrt{x}}{\sqrt{x}-3}< 0\)

Vì \(\sqrt{x}\ge0;\)\(6>0\)\(\Rightarrow6\sqrt{x}\ge0\)

\(\Rightarrow\frac{6\sqrt{x}}{\sqrt{x}-3}>0\Leftrightarrow\sqrt{x}-3< 0\)

\(\Rightarrow\sqrt{x}< 3\Rightarrow\sqrt{x}< \sqrt{9}\)\(\Leftrightarrow x< 9\)

Mà \(x\ge0\left(đkxđ\right)\)\(\Rightarrow0\le x< 9\)