Bài 4:
`a) (2-x)(x+3)=0`
`<=>`\(\left[ \begin{array}{l}2-x=0\\x+3=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=2\\x=-3\end{array} \right.\)
Vậy `x∈{2;-3}`
`b) -x(-2x-4)=0`
`<=> x(2x+4)=0`
`<=> 2x(x+2)=0`
`<=>`\(\left[ \begin{array}{l}x=0\\x+2=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=0\\x=-2\end{array} \right.\)
Vậy `x∈{0;-2}`
`c) (x^2+1)(-3x+9)=0`
Do `x^2+1>=1>0` với `∀x`
`=> 9-3x=0`
`<=> 3x=9`
`<=> x=3`
Vậy `x=3`
`d) (|x|+2)(x^2-4)=0`
Do `|x|+2>=2>0` với `∀x`
`=> x^2-4=0`
`<=> x^2=4`
`<=> x=+-2`
Vậy `x=+-2`
`e) (x-1)^2-16=0`
`<=> (x-1)^2=16`
`<=>`\(\left[ \begin{array}{l}x-1=4\\x-1=-4\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=5\\x=-3\end{array} \right.\)
Vậy `x∈{5;-3}`
`f) (1-3x)^3=-8`
`<=> (1-3x)^3=(-2)^3`
`<=> 1-3x=-2`
`<=> 3x=1+2`
`<=> 3x=3`
`<=> x=1`
Vậy `x=1`
