x4+x3+x2+x+1=0
⇔\(x^3\left(x+1\right)+x\left(x+1\right)+1=0\)
⇔\(\left(x+1\right)\left(x^3+x\right)+1=0\)
⇔\(\left(x+1\right)\left(x\left(x^2+1\right)\right)+1=0\)
⇔\(\left(x+1\right)\left(x\left(x+1\right)\left(x+1\right)\right)+1=0\)
⇔\(x+1=0\) hoặc \(x=0\)
⇔\(x=-1\) hoặc x=0