`f)`
`(x-1/3y)^3`
`=x^3-3.x^2 . 1/3y+3.x.(1/3y)^2-(1/3y)^3`
`=x^3-x^2y+1/3xy^2-1/27y^3`
`g)`
`(1/2x^2+1/3y)^3`
`=(1/2x^2)^3+3.(1/2x^2)^2 . 1/3y+3. 1/2x^2 .(1/3y)^2+(1/3y)^3`
`=1/8x^6+1/4x^4y+1/6x^2y^2+1/27y^3`
`h)`
`(2/3x^2-1/2y)^3`
`=(2/3x^2)^3-3.(2/3x^2)^2 . 1/2y+3 . 2/3x^2 . (1/2y)^2-(1/2y)^3`
`=8/27x^6-2/3x^4y+1/2x^2y^2-1/8y^3`
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Hằng đẳng thức:
`(A+B)^3=A^3+3A^2B+3AB^2+B^3`
`(A+B)^3=A^3-3A^2B+3AB^2-B^3`
