`1)3x^3+6x^2+3x-12x^2`
`=3x^3+(6x^2-12x^2)+3x`
`=3x^3-6x^2+3x`
`=3x(x^2-2x+1)`
`2)a^3b-ab^3+a^2+2ab+b^2`
`=a^3b-ab^3+(a^3+2ab+b^2)`
`=ab(a^2-b^2)+(a+b)^2`
`=ab(a-b)(a+b)+(a+b)(a+b)`
`=(a+b)[ab(a-b)+(a+b)]`
`=(a+b)(a^2b-ab^2+a+b)`
`3)(x^2-9y^2)-(4x+12y)`
`=[x^2-(3y)^2]-4(x+3y)`
`=(x+3y)(x-3y)-4(x+3y)`
`=(x+3y)(x-3y-4)`
`4)(x^2-16y^2)-3x+12y`
`=[x^2-(4y)^2]-(3x-12y)`
`=(x-4y)(x+4y)-3(x-4y)`
`=(x-4y)(x+4y-3)`
$\text{@ThanhTruc2008}$
