Đáp án:
Giải thích các bước giải:
a, -12 + 3.(-x + 7)=-18
⇔ 3.(-x + 7)=-18 + 12
⇔ 3.(-x + 7)=-6
⇔ -x + 7=-6 : 3
⇔ -x + 7=-2
⇔ -x =-2 - 7
⇔ -x =-9
⇔ x = 9
Vậy x=9
b,(-x + 31) - 39=-69 + 11
⇔-x + 31 - 39=-58
⇔-x - 8=-58
⇔-x =-58 + 8
⇔-x =-50
⇔ x =50
Vậy x=50
c,(x + 12).(x - 3)=0
⇔\(\left[ \begin{array}{l}x + 12=0\\x - 3=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x =-12\\x=3\end{array} \right.\)
Vậy x ∈ {-12;3}
d,x + 2021=|-2020| + |-5|
⇔x + 2021=-2020 + 5
⇔x + 2021=-2015
⇔x =-2015 - 2021
⇔x =-4036
Vậy x=-4036
e, |x - 7| + |-5|=9
⇔|x - 7| + 5=9
⇔|x - 7| =9 - 5
⇔|x - 7| =4
⇔\(\left[ \begin{array}{l}x-7=4\\x-7=-4\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=4+7\\x=-4+7\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=11\\x=3\end{array} \right.\)
Vậy x ∈ {11;3}
