`9)A=(x^2+x)^2+4x²+4x-12`
`=(x^2+x)^2+(4x²+4x)+4-16`
`=(x^2+x)^2+4(x²+x)+4-16`
`=[(x²+x)^2+4(x²+x)+4]-16`
`=[(x²+x)^2+2.(x²+x).2+2²]-16`
`=(x²+x+2)^2-4^2`
`=(x²+x+2-4)(x²+x+2+4)`
`=(x²+x-2)(x²+x+6)`
`=(x²+2x-x-2)(x²+x+6)`
`=[x(x+2)-(x+2)](x²+x+6)`
`=(x+2)(x-1)(x²+x+6)`
`10)A=(x²+x+1)^2+2x(x²+x+1)+x²`
`=(x²+x+1)^2+2.(x²+x+1).x+x²`
`=(x²+x+1+x)^2`
`=(x²+2x+1)^2`
`=[(x+1)^2]^2`
`=(x+1)^4`
Bài 3:
`1)x³-4x`
`=x(x²-4)`
`=x(x²-2²)`
`=x(x+2)(x-2)`
`2)x(x+y)-3x-3y`
`=x(x+y)-3(x+y)`
`=(x+y)(x-3)`
`3)x²-2x+1-y²`
`=(x²-2x+1)-y²`
`=(x-1)²-y²`
`=(x-1+y)(x-1-y)`
`4)2x³-8x`
`=2x(x²-4)`
`=2x(x²-2²)`
`=2x(x+2)(x-2)`
`5)x(x+y)-5x-5y`
`=x(x+y)-5(x+y)`
`=(x+y)(x-5)`
`6)x²-y²-2y-1`
`=x²-(y²+2y+1)`
`=x²-(y+1)²`
`=(x+y+1)(x-y-1)`
`7)2x²+6x`
`=2x.x+2x.3`
`=2x(x+3)`
`8)x(x+y)-7x-7y`
`=x(x+y)-7(x+y)`
`=(x+y)(x-7)`
`9)x²+2x-y²+1`
`=(x²+2x+1)-y²`
`=(x+1)²-y²`
`=(x+1+y)(x+1-y)`
`10)10x+15y`
`=5.2x+5.3y`
`=5(2x+3y)`
`11)x²(x-y)+2x-2y`
`=x²(x-y)+2(x-y)`
`=(x-y)(x²+2)`
`12)4a²-4a+1-b²`
`=(4a²-4a+1)-b²`
`=[(2a)²-2.2a.1+1²]-b²`
`=(2a-1)²-b²`
`=(2a-1+b)(2a-1-b)`
