Bài 13: Tìm x 3/. -12 + 3(-x + 7) = -18 Bài 14: Tìm x 2/. (x + 12) . (x - 3) = 0 3/. (-x + 5) . (3 - x) = 0 4/. x . (2 + x) . (7 - x) = 0 5/. (x - 1) . (x + 2) . (-x - 3) = 0 Bài 18: Chứng tỏ 1/. (a - b + c) - (a + c) = -b 2/. (a + b) - (b - a) + c = 2a + c 3/. - (a + b - c) + (a - b - c) = -2b 4/. a(b + c) - a(b + d) = a(c - d) 5/. a(b - c) + a(b + c) = a(b + d)

Bài `13.` $-12\:+\:3\left(-x\:+\:7\right)\:=\:-18$

$⇒3\left(7-x\right)-12=-18$

$⇒3\left(7-x\right)=-6$

$⇒7-x=-\dfrac{6}{3}$

$⇒7-x=-2$

$⇒-x=-9$

$⇒x=9$

`Vậy` $x=9$

Bài `14.`

$2/$ $\left(x+12\right)\left(x-3\right)=0$

\(⇒\left[ \begin{array}{l}x+12=0\\x-3=0\end{array} \right.\)

\(⇒\left[ \begin{array}{l}x=-12\\x=3\end{array} \right.\) 

`Vậy` $x\in $ $\left\{-12\:;\:3\right\}$

$3/$ $\left(-x+5\right)\left(3-x\right)=0$

\(⇒\left[ \begin{array}{l}-x+5=0\\3-x=0\end{array} \right.\)

\(⇒\left[ \begin{array}{l}x=5\\x=3\end{array} \right.\) 

`Vậy` $x\in $ $\left\{5\:;\:3\right\}$

$4/$ $x\left(2+x\right)\left(7-x\right)=0$

\(⇒\left[ \begin{array}{l}x=0\\2+x=0\\7-x=0\end{array} \right.\) 

\(⇒\left[ \begin{array}{l}x=0\\x=-2\\x=7\end{array} \right.\) 

`Vậy` $x\in $ $\left\{0\:;-2\:;\:7\right\}$

$5/$ $\left(x-1\right)\left(x+2\right)\left(-x-3\right)=0$

\(⇒\left[ \begin{array}{l}x-1=0\\x+2=0\\-x-3=0\end{array} \right.\) 

\(⇒\left[ \begin{array}{l}x=1\\x=-2\\x=-3\end{array} \right.\)

`Vậy` $x\in $ $\left\{1\:;-2\:;\:-3\right\}$ 

Bài `18.`

$1/$ $\left(a\:-\:b\:+\:c\right)\:-\:\left(a\:+\:c\right)\:=\:-b$

`VT=` $\left(a\:-\:b\:+\:c\right)\:-\:\left(a\:+\:c\right)\:$

$=a-b+c-\left(a+c\right)$

$=a-b+c-a-c$

$=a-a-b+c-c$

$=-b+c-c$

$=-b$ `=VP` `(đpcm)`

$2/$ $\left(a+b\right)-\left(b-a\right)+c=2a+c$

`VT=` $\left(a+b\right)-\left(b-a\right)+c$

$=a+b-\left(b-a\right)+c$

$=a+b-b+a+c$

$=a+a+c$

$=2a+c$ `=VP` `(đpcm)`

$3/$ $-\left(a+b-c\right)+\left(a-b-c\right)=-2b$

`VT=` $-\left(a+b-c\right)+\left(a-b-c\right)$

$=-\left(a+b-c\right)+a-b-c$

$=-a-b+c+a-b-c$

$=-a+a-b-b+c-c$

$=-b-b+c-c$

$=-2b+c-c$

$=-2b$ `=VP` `(đpcm)`

$4/$ $a\left(b+c\right)-a\left(b+d\right)=a\left(c-d\right)$

`VT=` $a\left(b+c\right)-a\left(b+d\right)$

$=ab+ac-a\left(b+d\right)$

$=ab+ac-ab-ad$

$=ab-ab+ac-ad$

$=ac-ad$

$=\:a\left(c\:-\:d\right)$ `=VP` `(đpcm)`

$5/$ $a\left(b-c\right)+a\left(d+c\right)=a\left(b+d\right)$

`VT=`  $a\left(b-c\right)+a\left(d+c\right)$

$=ab-ac+a\left(d+c\right)$

$=ab-ac+ad+ac$

$=ab-ac+ac+ad$

$=ab+ad$

$=a\left(b+d\right)$ `=VP` `(đpcm)`