\(a+b=2x-5\)

=>\(\left(a+b\right)^2=\left(2x-5\right)^2\)

=>\(a^2+b^2+2ab=4x^2-20x+25\)

=>\(2x^2+4x-1+2ab=4x^2-20x+25\)

=>\(2ab=2x^2-24x+26\)

=>\(ab=x^2-12x+13=x^2-12x+36-23=\left(x-6\right)^2-23\ge-23\).

\(ab\) đạt giá trị nhỏ nhất là -23 ⇔\(x-6=0\)⇔\(x=6\)

\(A=\left(\dfrac{1}{1-x}+\dfrac{2}{x+1}-\dfrac{5-x}{\left(1-x\right)\left(1+x\right)}\right):\dfrac{1-2x}{x^2-1}\)

\(A=\dfrac{x+1+2-2x-5+x}{\left(1-x\right)\left(1+x\right)}:\dfrac{1-2x}{x^2-1}\)

\(A=\dfrac{-2}{\left(1-x\right)\left(1+x\right)}.\dfrac{\left(x-1\right)\left(x+1\right)}{1-2x}\)

\(A=\dfrac{-2x+2}{1-2x}\)

\(A=0\)

⇔\(\dfrac{-2x+2}{1-2x}>0\)

⇔\(-2x+1>0\)

⇔\(-2x>-1\)

⇔\(x< \dfrac{1}{2}\)

Vậy x<\(\dfrac{1}{2}\) thì A>0

a: Khi x=1 thì\(P=\dfrac{1-2}{1+2}=\dfrac{-1}{2}\)

b: \(=\dfrac{3x+6+5x-6+2x^2-4x}{\left(x-2\right)\left(x+2\right)}=\dfrac{2x^2+4x}{\left(x-2\right)\left(x+2\right)}=\dfrac{2x}{x-2}\)

c: \(P=A\cdot B=\dfrac{2x}{x-2}\cdot\dfrac{x-2}{x+1}=\dfrac{2x}{x+1}\)

\(P-2=\dfrac{2x-2x-2}{x+1}=\dfrac{-2}{x+1}\)

P<=2

=>x+1>0

=>x>-1

a) Ta có: \(A=\dfrac{2x}{x+3}+\dfrac{x+1}{x-3}+\dfrac{3-11x}{9-x^2}\)

\(=\dfrac{2x\left(x-3\right)}{\left(x+3\right)\left(x-3\right)}+\dfrac{\left(x+1\right)\left(x+3\right)}{\left(x-3\right)\left(x+3\right)}+\dfrac{11x-3}{\left(x-3\right)\left(x+3\right)}\)

\(=\dfrac{2x^2-6x+x^2+4x+3+11x-3}{\left(x-3\right)\left(x+3\right)}\)

\(=\dfrac{3x^2+9x}{\left(x-3\right)\left(x+3\right)}\)

\(=\dfrac{3x\left(x+3\right)}{\left(x-3\right)\left(x+3\right)}=\dfrac{3x}{x-3}\)

b)

ĐKXĐ: \(x\notin\left\{3;-3;-1\right\}\)

Ta có: P=AB

\(=\dfrac{3x}{x-3}\cdot\dfrac{x-3}{x+1}\)

\(=\dfrac{3x}{x+1}\)

Để \(P=\dfrac{9}{2}\) thì \(\dfrac{3x}{x+1}=\dfrac{9}{2}\)

\(\Leftrightarrow9\left(x+1\right)=6x\)

\(\Leftrightarrow9x-6x=-9\)

\(\Leftrightarrow3x=-9\)

hay x=-3(loại)

Vậy: Không có giá trị nào của x để \(P=\dfrac{9}{2}\)

a: Khi x=2/3 thì \(A=\dfrac{\dfrac{2}{3}-2}{\dfrac{2}{3}}=\dfrac{-4}{3}\cdot\dfrac{3}{2}=-2\)

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

\(=\dfrac{4x^2-4x-x^2-x+2x}{\left(x-1\right)\left(x+1\right)}=\dfrac{3x^2-3x}{\left(x-1\right)\left(x+1\right)}=\dfrac{3x}{x+1}\)

a: \(A=\dfrac{x+2+x^2-2x+x-2}{\left(x-2\right)\left(x+2\right)}=\dfrac{x^2}{x^2-4}\)

a: \(A=\dfrac{x+2+x^2-2x+x-2}{\left(x-2\right)\left(x+2\right)}=\dfrac{x^2-2x}{\left(x-2\right)\left(x+2\right)}=\dfrac{x}{x+2}\)