\(a,\Leftrightarrow\left(2-x\right)\left(x^2+4\right)>0\Leftrightarrow2-x>0\Leftrightarrow x< 2\\ b,\Leftrightarrow x+3>0\Leftrightarrow x>-3\\ c,\Leftrightarrow\left[{}\begin{matrix}x< -3\\x>4\end{matrix}\right.\)

b: \(\Leftrightarrow x+3>0\)

hay x>-3

a) x(x+3)=0

TH1: x=0                            TH2:x+3=0

                                                      x= -3

b)(x-2)(5-x)=0

TH1: x-2=0                         TH2: 5-x=0

             x= -2                                  x=5

c)làm tương tự những câu trên 

chúc bạn học tốt

a, x(x+3)=0

<=> x=0 hoặc x+3=0

<=>x=0 hoặc x=-3

Vậy \(x\in\left\{0;-3\right\}\)

b, (x-2)(5-x)=0

<=>x-2=0 hoặc 5-x=0

<=>x=2 hoặc x=5

Vậy \(x\in\left\{2;5\right\}\)

c,(x-1)(x² +1)=0

<=> x-1=0 hoặc x²+1=0

<=>x=1 

Vậy x=1

a. \(x\left(x+3\right)=0\)

\(\Leftrightarrow\hept{\begin{cases}x=0\\x+3=0\end{cases}}\Rightarrow\hept{\begin{cases}x=0\\x=-3\end{cases}}\)

b. \(\left(x-2\right)\left(5-x\right)=0\)

\(\Leftrightarrow\hept{\begin{cases}x-2=0\\5-x=0\end{cases}}\Rightarrow\hept{\begin{cases}x=2\\x=5\end{cases}}\)

c. \(\left(x-1\right)\left(x^2+1\right)=0\)

\(\Leftrightarrow\hept{\begin{cases}x-1=0\\x^2+1=0\end{cases}}\Rightarrow\hept{\begin{cases}x=1\\x^2=-1\left(ktm\right)\end{cases}}\)

a) x Î{0;3}.

b) xÎ{0;-9}.

c) x Î{-l; 11}.

d) x = 13.

\((x-6)(3x-9)>0\)
TH1:
\(\orbr{\begin{cases}x-6< 0\\3x-9< 0\end{cases}}\)\(\Rightarrow\orbr{\begin{cases}x< 6\\x< 3\end{cases}}\)\(\Rightarrow x< 3\)
TH2:
\(\orbr{\begin{cases}x-6>0\\3x-9>0\end{cases}}\)\(\Rightarrow\orbr{\begin{cases}x>6\\x>3\end{cases}}\)\(\Rightarrow x>6\)
Vậy \(x< 3\) hoặc \(x>6\)thì \((x-6)(3x-9)>0\)
Học tốt!

20.
\((2x-1)(6-x)>0\)
TH1:
\(\orbr{\begin{cases}2x-1>0\\6-x>0\end{cases}\Rightarrow\orbr{\begin{cases}x< \frac{1}{2}\\x< 6\end{cases}}\Rightarrow x< 6}\)
TH2
\(\orbr{\begin{cases}2x-1< 0\\6-x< 0\end{cases}\Rightarrow\orbr{\begin{cases}x>\frac{1}{2}\\x>6\end{cases}}\Rightarrow x>\frac{1}{2}}\)
Vậy \(x< 6\)hoặc \(x>\frac{1}{2}\)thì \((2x-1)(6-x)>0\)
 

21.
\((2-x)(x+7)< 0\)
TH1.
\(\orbr{\begin{cases}2-x>0\\x+7< 0\end{cases}\Rightarrow\orbr{\begin{cases}x< 2\\x>-7\end{cases}}\Rightarrow-7< x< 2}\)
TH2.
\(\orbr{\begin{cases}2-x< 0\\x+7>0\end{cases}\Rightarrow\orbr{\begin{cases}x>2\\x< -7\end{cases}}\Rightarrow2< x< -7}\)(vô lí)
Vậy \(-7< x< 2\) thì \((2-x)(x+7)< 0\)
 

a) x = 1; x = - 1 3                 b) x = 2.

c) x = 3; x = -2.                 d) x = -3; x = 0; x = 2.

a) \(\Rightarrow x\left(x+3\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=0\\x=-3\end{matrix}\right.\)

b) \(\Rightarrow x\left(x^2-4\right)=0\Rightarrow x\left(x-2\right)\left(x+2\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=0\\x=2\\x=-2\end{matrix}\right.\)

c) \(\Rightarrow\left(x-1\right)\left(5x-1\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=1\\x=\dfrac{1}{5}\end{matrix}\right.\)

d) \(\Rightarrow2\left(x+5\right)-x\left(x+5\right)=0\Rightarrow\left(x+5\right)\left(2-x\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=-5\\x=2\end{matrix}\right.\)

e) \(\Rightarrow2x^2-10x-3x-2x^2=26\)

\(\Rightarrow-13x=26\Rightarrow x=-2\)

f) \(\Rightarrow\left(x-2012\right)\left(5x-1\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=2012\\x=\dfrac{1}{5}\end{matrix}\right.\)