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Bài `4:`

`a)26-(x+12)=-15`

`=>x+12=26-(-15)`

`=>x+12=26+15`

`=>x=41-12`

`=>x=29`

`b)x-(-15)=-13-(-85-13)`

`=>x+15=-13+85+13`

`=>x+15=85`

`=>x=85-15`

`=>x=70`

`c)123-5(x+4)=38`

`=>5(x+4)=123-38`

`=>5(x+4)=85`

`=>x+4=85:5`

`=>x+4=17`

`=>x=13`

`d)25+(x-5)=-123-(15-123)`

`=>25+x-5=-123-15+123`

`=>20+x=-15`

`=>x=-15-20`

`=>x=-35`

`e)(24-x)(15+x)=0`

`<=>` \(\left[ \begin{array}{l}24-x=0\\15+x=0\end{array} \right.\)

`<=>` \(\left[ \begin{array}{l}x=24\\x=-15\end{array} \right.\) 8888888

`f)(x-5)(x^2-9)=0`

`<=>` \(\left[ \begin{array}{l}x-5=0\\x^2-9=0\end{array} \right.\) 

`<=>` \(\left[ \begin{array}{I}x=5\\x^2=9\end{array} \right.\)

`<=>` \(\left[ \begin{array}{l}x=5\\x^2=3^2\end{array} \right.\) 

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