x³+x²+4x+4=0
x²(x+1)+4(x+1)=0
(x+1)(x²+4)=0
x+1=0 sau x²+4=0
x=-1 sau x=-2i sau x=2i
[tex] \frac{1}{ x_{1} } + \frac{1}{ x_{2} }+ \frac{1}{ x_{3} } + \frac{1}{ x_{1} x_{2} } + \frac{1}{ x_{1} x_{3} }+ \frac{1}{ x_{2} x_{3} } = \frac{1}{-1} + \frac{1}{-2i}+ \frac{1}{2i}+ \\ + \frac{1}{-2i}+ \frac{1}{2i}+ \frac{1}{4}=-1+ \frac{1}{4}=- \frac{3}{4} [/tex]
