Bài 1)
a) 5x - 10 = 0
⇒5x=10
⇒x=$\frac{10}{5}$ =2
b)(2x-3).(x+7)=0
⇒$\left \{ {{2x-3=0} \atop {x+7=0}} \right.$
⇒$\left\{{{x=\frac{3}{2}} \atop {x=-7}} \right.$
c)$\frac{1}{x+4}$ +$\frac{3x}{(x+4)(x-2)}$ =$\frac{3}{x-2}$
⇔$\frac{(x-2)+3x}{(x+4)(x-2)}$ = $\frac{3(x+4)}{(x+4)(x-2)}$
⇔(x-2) + 3x = 3
⇔4x - 5 = 0
⇔x=$\frac{5}{4}$
d) l2x - 3l=-x+21
⇔$\left \{ {{2x-3=±(-x+21)} \atop {-x+21\geq0}} \right.$
⇔$\left \{ {{2x-3=-x+21 ;2x-3 =x-21} \atop {x\leq21}} \right.$
⇔$\left \{ {{x=8(nhận);x=-18(nhận)} \atop {x\leq21}} \right.$
⇔$\left \{ {{x=8} \atop {x=-18}} \right.$
