a: Khi x=3 thì \(A=\dfrac{3+3}{9-4}=\dfrac{6}{5}\)
b: \(B=\dfrac{x}{\left(x-2\right)\left(x+2\right)}-\dfrac{1}{x-2}-\dfrac{x}{x+2}\)
\(=\dfrac{x-x-2-x^2+2x}{\left(x-2\right)\left(x+2\right)}\)
\(=\dfrac{-x^2+2x-2}{\left(x-2\right)\left(x+2\right)}\)