`27x^3 - 27x^2y+ 9xy^2 - y^3 = (3x)^3 - 3.(3x)^2 .y + 3.3x . y^2 - y^3 = (3x-y)^3`
`(a+b)(a^3+b^3) - (a-b)(a^3+b^3) = (a^3+b^3)(a+b-a+b) = 2b(a^3+b^3) `
`(1-x-2x^3+3x^2)(1-x+2x^3-3x^2) = [(1-x) - (2x^3-3x^2)]`
`= (1-x)^2 - (2x^3-3x^2)^2`
`= x^2-2x+1 - 4x^6 + 12x^5 - 9x^4`