Lời giải :
`b)2x^2y-6xy^2=2xy(x-3y)`
`d)6x^4-9x^3=3x^3(2x-3)`
`f)9x^2y^2+15x^2y-21xy^2=3xy(3xy+5x-7y)`
`h)x^2-6xy+9y^2=x^2-2.3y.x+(3y)^2=(x-3y)^2`
`k)x^3-64=x^3-4^3=(x-4)(x^2+x+16)`
`m)2x(x+1)+2(x+1)=(2x+2)(x+1)=2(x+1)(x+1)=2(x+1)^2=2(x^2+2x+1)`
`o)4x(x-2y)+8y(2y-x)=4x(x-2x)-8y(x-2y)=(4x-8y)(x-2y)=4(x-2y)(x-2y)=4(x-2y)^2=4(x^2-4xy+4t^2)`
`q)(2x+1)^2-(x-1)^2=4x^2+5x+1-x^2+2x-1=3x^2+7x=x(3x+7)`
`s)x^4+x^3+x+1=(x^4+x^3)+(x+1)=(x^3+1)(x+1)=(x+1)(x^2-x+1)(x+1)=(x+1)^2(x^2-x+1)=(x^2+2x+1)(x^2-x+1)`
`u)x^2-2xy+y^2-xz+yz=(x^2-2xy+y^2)-z(x-y)=(x-y)^2-z(x-y)=(x-y)(x-y-z)`
