\(\frac{x^3+x^2+x+1}{3x^2+6x+3}=\frac{x^2\left(x+1\right)+\left(x+1\right)}{3\left(x^2+2x+1\right)}=\frac{\left(x^2+1\right)\left(x+1\right)}{3\left(x^2+x+x+1\right)}=\frac{\left(x^2+1\right)\left(x+1\right)}{3\left[x\left(x+1\right)+\left(x+1\right)\right]}\)
\(=\frac{\left(x^2+1\right)\left(x+1\right)}{3\left(x+1\right)\left(x+1\right)}=\frac{x^2+1}{3\left(x+1\right)}\)
nè bn thành cái chỗ X2+2x+1 đã là HĐT rồi thì tách làm j nữa dài đó