a: ĐKXĐ: x<>1; x<>-1
b: \(A=\left(\dfrac{1}{x^2-x+1}+\dfrac{1}{x+1}\right)\cdot\dfrac{x+1}{x^2+2}\)
\(=\dfrac{x+1+x^2-x+1}{\left(x+1\right)\left(x^2-x+1\right)}\cdot\dfrac{x+1}{x^2+2}=\dfrac{1}{x^2-x+1}\)
c: x^2-x+1=(x-1/2)^2+3/4>=3/4
=>A<=4/3
Dấu = xảy ra khi x=1/2