11: \(\Leftrightarrow x^2-2x+3=x+3\)
\(\Leftrightarrow x\left(x-3\right)=0\)
hay x=3
12: \(\Leftrightarrow\left(x-1\right)\left(x-2012\right)=0\)
hay \(x\in\left\{1;2012\right\}\)
13: \(\Leftrightarrow\left(3x-1\right)\left(x+3\right)-\left(2x+5\right)\left(x-1\right)=\left(x-1\right)\left(x+3\right)-4\)
\(\Leftrightarrow3x^2+9x-x-3-2x^2+2x-5x+5=x^2+2x-3-4\)
\(\Leftrightarrow x^2+5x+2-x^2-2x+7=0\)
=>3x+9=0
hay x=-3(loại)
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