1 câu trả lời
`f(x)=(x-4)(-x^2+9)`
`f(x)=-(x-4)(x^2-9)`
`f(x)=(4-x)(x-3)(x+3)`
Xét `4-x=0<=>x=4`
`x-3=0<=>x=3`
`x+3=0<=>x=-3`
BXD:
\begin{array}{|c|cc|}\hline x&-\infty & &-3&&3&&4&&+\infty&\\\hline 4-x & &+ & |&+&|&+&0&-&&\\\hline x-3& &- &|&-&0&+&|&+&&\\\hline x+3&&-&0&+&|&+&|&+&&\\\hline f(x)&&+&0&-&0&+&0&-\\\hline\end{array}
`KL: @f(x) > 0<=>x \in (-oo;-3)uu(3;4)`
`@f(x) < 0<=>x \in (-3;3)uu(4;+oo)`
`@f(x) = 0<=>x = {-3;3;4}`
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