a) (2× x -8)×(1-x)=0 b) ( 15-3× x)×(x-6)=0 c) (x+1) +(x+2)+ +(x+20)=230 d) 56-3×x+10×x=140

a) `(2x-8)(1-x)=0`

`⇒` \(\left[ \begin{array}{l}2x-8=0\\1-x=0\end{array} \right.\) 

`⇒` \(\left[ \begin{array}{l}2x=8\\x=1\end{array} \right.\) 

`⇒` \(\left[ \begin{array}{l}x=4\\x=1\end{array} \right.\)

b) `(15-3x)(x-6)=0`

`⇒` \(\left[ \begin{array}{l}15-3x=0\\x-6=0\end{array} \right.\)  

`⇒` \(\left[ \begin{array}{l}3x=15\\x=6\end{array} \right.\)

`⇒` \(\left[ \begin{array}{l}x=5\\x=6\end{array} \right.\)

c) `(x+1)+(x+2)+…+(x+20)=230`

`⇒ x+1+x+2+…+x+20=320`

`⇒ (x+x+…+x)+(1+2+…+20)=230`

`⇒ 20x+210=230`

`⇒ 20x=20`

`⇒ x=1`

d) `56-3x+10x=140`

`⇒ 56+7x=140`

`⇒ 7x=84`

`⇒ x=12`