`1.`
`(x-2)^2=x`
`⇒x^2-4x+4-x=0`
`⇒x^2-5x+4=0`
`⇒x^2-x-4x+4=0`
`⇒x(x-1)-4(x-1)=0`
`⇒(x-4)(x-1)=0`
`⇒` \(\left[ \begin{array}{l}x-4=0\\x-1=0\end{array} \right.\)
`⇒` \(\left[ \begin{array}{l}x=4\\x=1\end{array} \right.\)
Vậy `x∈{4;1}`
`2.`
`a)x^2-y^2+6x+9`
`=(x^2+6x+9)-y^2`
`=(x^2+2.x.3+3^2)-y^2`
`=(x+3)^2-y^2`
`=(x+3-y)(x+3+y)`
`b)4x^2-25`
`=(2x)^2-5^2`
`=(2x-5)(2x+5)`