Actually, your equation
x³+2x²+x≥0
is a cubic equation, since the highest-order term is x³. Fortunately, we can easily factor out an overall x,
x(x²+2x+1) ≥0,
and write the quadratic term as a complete square:
x(x+1)² ≥0.
How can the left-hand side be non-negative (which is what "≥0" means)?
Well, the term (x+1)² is always non-negative, because it's a square. In fact it is
positive for all x≠-1, only for x=-1 is it zero.
So the entire left-hand side of the inequality is non-negative when x is non-negative, i.e. for all x≥0, and when x=-1. In interval notation, you could write the solution set as
{-1}∪[0, ∞).
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