`11)9x-x^3`
`=x(9-x^2)`
`=x(3-x)(3+x)`
`12)8x^3-27`
`=(2x)^3-3^3`
`=(2x-3)(4x^2+6x+9)`
`13)x^3-3x^2+3x-1`
`=x^3-3.x^2.1+3.x.1^2-1`
`=(x-1)^3`
`14)8x^3+36x^2-54x-27`
`=8x^3+48x^2+18x-12x^2-72x-27`
`=(2x-3)(4x^2+24x+9)`
`15)25(x+y)^2-16(x-y)^2`
`=(5(x+y))^2-(4(x-y))^2`
`=(5x+5y-4x+4y)(5x+5y+4x-4y)`
`=(x+9y)(9x+y)`
`16)x^6-1`
`=(x^3-1)(x^3+1)`
`=(x-1)(x^2+x+1)(x+1)(x^2-x+1)`
`17)(x+y)^3+(x-y)^3`
`=(x+y+x-y)(x^2+2xy+y^2-x^2+y^2+x^2-2xy+y^2)`
`=2x(x^2+3y^2)`
`18)6x-9-x^2`
`=-x^2+6x-9`
`=-(x^2-2.3.x+3^2)`
`=-(x-3)^2`
