đã là ng thik roii thì cái j cx là gu keke^^

\(\sqrt{4x+9}-3=0\\ \Leftrightarrow\sqrt{4x+9}=3\\ \Leftrightarrow\sqrt{\left(4x+9\right)^2}=3^2\\ \Leftrightarrow4x+9=9\\ \Leftrightarrow4x=0\\ \Leftrightarrow x=0\)

`a,` Khi `x=9` Ta có : 

\(A=\dfrac{\sqrt{x}+3}{x-4}\\ =\dfrac{\sqrt{9}+3}{9-4}\\ =\dfrac{3+3}{5}\\ =\dfrac{6}{5}\)

\(b,B=\dfrac{-4}{x-4}+\dfrac{1}{\sqrt{x}-2}\\ =-\dfrac{4}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}+\dfrac{1}{\sqrt{x}-2}\\ =-\dfrac{4}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}+\dfrac{\sqrt{x}+2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\\ =\dfrac{-4+\sqrt{x}+2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\\ =\dfrac{\sqrt{x}-2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\\ =\dfrac{1}{\sqrt{x}+2}\)

`c,` `P=A/B`

\(\Rightarrow P=\dfrac{\dfrac{\sqrt{x}+3}{x-4}}{\dfrac{1}{\sqrt{x}+2}}\\ =\dfrac{\sqrt{x}+3}{x-4}\cdot\dfrac{\sqrt{x}+2}{1}\\ =\dfrac{\sqrt{x}+3}{\sqrt{x}-2}\)

Mà `P<1` thì :

\(\dfrac{\sqrt{x}+3}{\sqrt{x}-2}< 1\\ \Leftrightarrow\dfrac{\sqrt{x}+3}{\sqrt{x}-2}< \dfrac{\sqrt{x}-2}{\sqrt{x}-2}\\ \Leftrightarrow\sqrt{x}+3< \sqrt{x}-2\\ \Leftrightarrow0< -5\)

Vậy ko có `x` thỏa mãn `P<1`

`220 -20(x+5)=60`

`=> 20(x+5)=220-60`

`=> 20(x+5) = 160`

`=> x+5=160:20`

`=> x+5=8`

`=> x=8-5`

`=>x=3`

Vậy `x=3`

Tự luận.

Bài `1`

`a,M=A+B`

`=x^3-x^2-2x+1+(-x^3)+x^2`

`= (x^3-x^3)+(-x^2+x^2) -2x+1`

`= -2x+1`

`b,` Tại `x=1`

Ta có : `-2x+1=-2.1+1= -2+1=-1`

Bài `2`

\(\left(x+1\right)\left(x-1\right)=0\\ \Rightarrow\left[{}\begin{matrix}x+1=0\\x-1=0\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}x=-1\\x=1\end{matrix}\right.\)

Bài `1`

\(a,5\sqrt{\dfrac{1}{5}}+\dfrac{1}{2}\sqrt{20}-\sqrt{45}\\ =\sqrt{\dfrac{25}{5}}+\dfrac{1}{2}\sqrt{4\cdot5}-\sqrt{9\cdot5}\\ =\sqrt{5}+\sqrt{5}-3\sqrt{5}\\ =-\sqrt{5}\)

\(b,\dfrac{2}{2-\sqrt{3}}-\dfrac{1}{2+\sqrt{3}}-3\sqrt{4-2\sqrt{3}}\\ =\dfrac{2\left(2+\sqrt{3}\right)}{4-3}-\dfrac{2-\sqrt{3}}{4-3}-3\sqrt{3-2\sqrt{3}+1}\\ =\left(4+2\sqrt{3}-2+\sqrt{3}\right)-3\sqrt{\left(\sqrt{3}-1\right)^2}\\ =\left(2+3\sqrt{3}\right)-3\left(\sqrt{3}-1\right)\\ =2+\sqrt{3}-3\sqrt{3}+3\\ =5-2\sqrt{3}\)

bài `2`

\(a,\sqrt{x^2-4x+4}=5\\ \Leftrightarrow\sqrt{\left(x-2\right)^2}=5\\ \Leftrightarrow x-2=5\\ \Leftrightarrow x=7\)

\(b,\sqrt{4x+20}-3\sqrt{x+5}+\sqrt{16x+80}=15\\ \Leftrightarrow\sqrt{4\left(x+5\right)}-3\sqrt{x+5}+\sqrt{16\left(x+5\right)}=15\\ \Leftrightarrow2\sqrt{x+5}-3\sqrt{x+5}+4\sqrt{x+5}=15\\ \Leftrightarrow3\sqrt{x+5}=15\\ \Leftrightarrow\sqrt{x+5}=5\\ \Leftrightarrow x+5=25\\ \Leftrightarrow x=20\)