a/
`(x+4)(x^2 -4x+16)`
`= (x+4)(x^2 - 4x + 4^2)`
`= x^3 +4^3`
`= x^3 +64`
b/
`(x-3y)(x^2+3xy +9y^2)`
`= (x-3y)[(x^2 + 3xy +(3y)^2 ]`
`= x^3 - (3y)^3`
`= x^3 -27y^3`
c/
`(1/3x+2y)(1/9x^2-2/3xy +4y^2)`
`= (1/3x+2y)[(1/3x)^2 - 1/3x . 2y + (2y)^2]`
`= (1/3x)^3 +(2y)^3`
`= 1/27x^3 +8y^3`
d/
`(x^2 -1/3)(x^4 +1/3x^2+1/9)`
`= (x^2 -1/3)[ (x^2)^2 + 1/3 . x^2 +(1/3)^2 ]`
`= (x^2)^3 -(1/3)^3`
`= x^6 -1/27`
