`f(x)=28x^3-3x^2y+3xy^2-y^3`
` =27x^3+x^3-3x^2y+3xy^2-y^3`
` =(3x)^3+(x-y)^3`
` =(3x+x-y)[(3x)^2-3x(x-y)+(x-y)^2]`
` =(4x-y)(9x^2-3x^2+3xy+x^2-2xy+y^2)`
` =(4x-y)(7x^2+xy+y^2)`
`*`Áp dụng hằng đẳng thức
`(a-b)^3=a^3-2a^2b+3ab^2-b^3`
`a^3+b^3=(a+b)(a^2-ab+b^2)`