a/ đkxđ: x \(\ne\pm\)2; x≠3
\(P=\left(\dfrac{2+x}{2-x}-\dfrac{2-x}{2+x}-\dfrac{4x^2}{x^2-4}\right):\dfrac{x^2-6x+9}{\left(2-x\right)\left(x-3\right)}\)
\(=\left(\dfrac{\left(2+x\right)^2-\left(2-x\right)^2}{\left(2-x\right)\left(2+x\right)}+\dfrac{4x^2}{x^2-4}\right):\dfrac{\left(x-3\right)^2}{\left(2-x\right)\left(x-3\right)}\)
\(=\dfrac{x^2+4x+4-x^2+4x-4+4x^2}{\left(2-x\right)\left(2+x\right)}\cdot\dfrac{2-x}{x-3}\)
\(=\dfrac{8x+4x^2}{2+x}\cdot\dfrac{1}{x-3}=\dfrac{4x\left(2+x\right)}{2+x}\cdot\dfrac{1}{x-3}=\dfrac{4x}{x-3}\)
b/ x = \(\dfrac{1}{3}\Leftrightarrow P=\dfrac{4\cdot\dfrac{1}{3}}{\dfrac{1}{3}-3}=\dfrac{4}{3}:\left(-\dfrac{8}{3}\right)=\dfrac{4}{3}\cdot\left(-\dfrac{3}{8}\right)=-\dfrac{4}{8}=-\dfrac{1}{2}\)
c/ \(P\in Z\Rightarrow\dfrac{4x}{x-3}\in Z\)
Ta có: \(\dfrac{4x}{x-3}=\dfrac{4x-12+12}{x-3}=\dfrac{4\left(x-3\right)}{x-3}+\dfrac{12}{x-3}=4+\dfrac{12}{x-3}\)
=> \(x-3\inƯ\left(12\right)\) thì P ∈ Z
=> \(x-3=\left\{-12;-6;-4;-3;-2;-1;1;2;3;4;6;12\right\}\)
\(\Leftrightarrow x=\left\{-9;-3;-1;0;1;2;4;5;6;7;9;15\right\}\)
mà x>4
=> x = {5;6;7;9;15}
a, Ta có:
\(P=\left(\dfrac{2+x}{2-x}-\dfrac{2-x}{2+x}-\dfrac{4x^2}{x^2-4}\right):\dfrac{x^2-6x+9}{\left(2-x\right)\left(x-3\right)}\)
\(=\left(\dfrac{2+x}{2-x}-\dfrac{2-x}{2+x}+\dfrac{4x^2}{4-x^2}\right):\left[\dfrac{\left(x-3\right)^2}{\left(2-x\right)\left(x-3\right)}\right]\)
\(=\left(\dfrac{2+x}{2-x}-\dfrac{2-x}{2+x}+\dfrac{4x^2}{\left(2-x\right)\left(2+x\right)}\right):\dfrac{x-3}{2-x}\)
\(=\dfrac{\left(2+x\right)^2-\left(2-x\right)^2+4x^2}{\left(2-x\right)\left(2+x\right)}.\dfrac{2-x}{x-3}\)
\(=\dfrac{4+4x+x^2-\left(4-4x+x^2\right)+4x^2}{\left(2-x\right)\left(2+x\right)}.\dfrac{2-x}{x-3}\)
\(=\dfrac{4+4x+x^2-4+4x-x^2+4x^2}{\left(2-x\right)\left(2+x\right)}.\dfrac{2-x}{x-3}\)
\(=\dfrac{4x^2+8x}{\left(2-x\right)\left(2+x\right)}.\dfrac{2-x}{x-3}\)
\(=\dfrac{4x\left(x+2\right)}{\left(2-x\right)\left(2+x\right)}.\dfrac{2-x}{x-3}\)
\(=\dfrac{4x}{x-3}\)
