$c)ĐK:x \ne -1,x \ne -2\\PT\Leftrightarrow \dfrac{5x}{x+2}+1+\dfrac{6}{x+1}=0\\\Leftrightarrow 5x(x+1)+(x+1)(x+2)+6(x+2)=0\\\Leftrightarrow 5x^2+5x+x^2+3x+2+6x+12=0\\\Leftrightarrow 6x^2+14x+14=0\\\Leftrightarrow x^2+\dfrac73x+\dfrac73=0\\\Leftrightarrow x^2+\dfrac73x+\dfrac{49}{36}+\dfrac{35}{36}=0\\\Leftrightarrow (x+\dfrac76)^2=-\dfrac{35}{36}$ vô lý
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