a)
\(\dfrac{1}{x+3}+\dfrac{1}{x-3}+\dfrac{1}{x^2-9}\)
\(=\dfrac{1}{x+3}+\dfrac{1}{x-3}+\dfrac{1}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{1\left(x-3\right)}{\left(x+3\right)\left(x-3\right)}+\dfrac{1\left(x+3\right)}{\left(x-3\right)\left(x+3\right)}+\dfrac{1}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{x-3+x+3+1}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{x-x-3+3+1}{\left(x-3\right)\left(x+3\right)}\)
\(=\dfrac{1}{\left(x-3\right)\left(x+3\right)}\)
c)
\(\dfrac{1}{x-1}+\dfrac{1}{x^2+x+1}+\dfrac{1}{x^3-1}\)
\(=\dfrac{1}{x-1}+\dfrac{1}{x^2+x+1}+\dfrac{1}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\dfrac{1\left(x^2+x+1\right)}{\left(x-1\right)\left(x^2+x+1\right)}+\dfrac{1\left(x-1\right)}{\left(x-1\right)\left(x^2+x+1\right)}+\dfrac{1}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\dfrac{x^2+x+1+x-1+1}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\dfrac{x^2+x+x+1-1+1}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\dfrac{x^2+2x+1}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(=\dfrac{\left(x-1\right)^2}{\left(x-1\right)\left(x^2+x+1\right)}\)
\(\dfrac{x-1}{x^2+x+1}\)
giúp mk vs ạ ai nhanh mk tick nha 
