x^3+2x^2+x+2=0
x^2(x+2)+(x+2)=0
(x^2+1)(x+2)=0
x^2+1=0无解
x=-2
f(x)=(x+90)(x^2-56x+5408)=0
x^2-56x+5408=0
判别式小于0,无解
所以x=-90