`1)(x^2-1)/(x(x-1))`
`=((x-1)(x+1))/(x(x-1))`
`=(x+1)/x`
`→C`
`2)(x^2-2x+1):(x-1)`
`=(x-1)^2:(x-1)`
`=x-1`
`→B`
$3)ĐKXĐ:x^2+2x\ne0⇔x(x+2)\ne0$
`⇔x\ne0;x\ne-2`
`→D`
$4)ĐKXĐ:x^2-4\ne0⇔(x-2)(x+2)\ne0$
`⇔x-2\ne0;x+2\ne0`
`⇔x\ne2;x\ne-2`
`→C`
`5)x^2-2xy+y^2-1`
`=(x-y)^2-1^2`
`=(x-y-1)(x-y+1)`
`→A`
`6)(x+y)^2-(x-y)^2`
`=x^2+2xy+y^2-(x^2-2xy+y^2)`
`=x^2+2xy+y^2-x^2+2xy-y^2`
`=(x^2-x^2)+(2xy+2xy)+(y^2-y^2)`
`=4xy`
`→C`