9: =>12x=42x2,6
=>x=7x1,3=9,1
10: =>x2=36
=>x=6 hoặc x=-6
19: =>7x-7=6x-30
=>x=-23
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10, \(x^2=36\Leftrightarrow x=6;x=-6\)
11, \(\dfrac{7}{5}x=\dfrac{21}{10}\left(-5\right)=-\dfrac{21}{2}\Leftrightarrow x=-\dfrac{21}{2}:\dfrac{7}{5}=-\dfrac{3}{10}\)
13, \(5x-10=12\Leftrightarrow x=\dfrac{22}{5}\)
14, \(20x+60=-20\Leftrightarrow20x=-80\Leftrightarrow x=-4\)