a, \(\left(x+3\right)\left(x-4\right)< 0\)
\(\Rightarrow x^2-x-12< 0\)
\(\Rightarrow\left(x-0,5\right)^2< 12,25\)
\(\Rightarrow3,5>x-0,5>-3,5\)
\(\Rightarrow4>x>-3\)
b,\(\Rightarrow\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+...+\frac{2}{x\left(x+1\right)}=\frac{2012}{2014}\)
\(\Rightarrow2.\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2012}{2014}\)
\(\Rightarrow2.\frac{x-1}{2x+2}=\frac{2012}{2014}\)
\(\Rightarrow\frac{x-1}{x+1}=\frac{2012}{2014}\Rightarrow x=2013\)
chúc bạn học tốt ^^
\(\left(x+3\right)\left(x-4\right)< 0\)
Ta có 2 trường hợp
Trường hợp 1:
\(PT\Leftrightarrow\hept{\begin{cases}x+3>0\\x-4< 0\end{cases}\Leftrightarrow\hept{\begin{cases}x>-3\\x< 4\end{cases}}}\)
\(\Rightarrow x< 4\left(1\right)\)
Trường hợp 2:
\(PT\Leftrightarrow\hept{\begin{cases}x+3< 0\\x-4>0\end{cases}\Leftrightarrow\hept{\begin{cases}x< -3\\x>4\end{cases}}}\)
\(\Rightarrow x< -3\left(2\right)\)
Từ (1) và (2)
\(\Rightarrow4>x< -3\)
Vậy \(x\in\){-4;-5;-6;-7-;-8;.....}