`16, (x + 3y)^3`
`= x^3 + 3. x^2. 3y + 3. x. (3y)^2 + (3y)^3`
`= x^3 + 9x^2y + 27xy^2+ 27y^3`
`17, (1/3x + 2)^2`
`= (1/3x)^2 + 2. 1/3x. 2 + 2^2`
`= 1/9x^2 + 4/3x + 4`
`18, (x - 2)(x^2 + 2x + 4)`
`= (x - 2)(x^2 + 2x + 2^2)`
`= x^3 - 2^3`
`= x^3 - 8`
`19, 64x^3 - 8`
`= (4x)^3 - 2^3`
`= (4x - 2)(16x^2 + 8x + 4)`
`20, (1/2x + 1/3y)(1/4x^2 - 1/6xy + 1/9y^2)`
`= (1/2x + 1/3y)[(1/2x)^2 - 1/2x. 1/3 + (1/3y)^2]`
`= (1/2x)^3 - (1/3y)^3`
`= 1/8x^3 - 1/27y^3`
