Bài 3:
`(x+1)^2+2(1-x)`
`=x^2+2x+1+2-2x`
`=x^2+3`
Bài 4:
`a) x^2-4=9(x-2)`
`<=> (x-2)(x+2)-9(x-2)=0`
`<=> (x-2)(x+2-9)=0`
`<=> (x-2)(x-7)=0`
`<=>`\(\left[ \begin{array}{l}x-2=0\\x-7=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=2\\x=7\end{array} \right.\)
`b) 2(x^2+1)-2x(x-1)=0`
`<=> 2x^2+2-2x^2+2x=0`
`<=> 2x+2=0`
`<=> x+1=0`
`<=> x=-1`