Question

Solve. x^4+8x^3+21x^2+18x is less than or equal to 0

Answer 1

\[x^4+8x^3+21x^2+18x \le 0\]

Answer 2

Notice that
\[
x^4+8 x^3+21 x^2+18 x=x (x+2)
(x+3)^2
\]
It is enough to see when
\[
x (x+2) <0
\]

Answer 3

This happens if
-2 < x < 0

Answer 4

how did you factor it like that?

Answer 5

and how come (x+3)^2 is not less than 0?