`(x^2y^2 - 1/2xy +2y).(x-2y)`
`= x.( x^2y^2 - 1/2xy +2y) + 2y . (x^2y^2 - 1/2xy +2y)`
` = x^3y^2 - 1/2x^2y +2xy - 2x^2y^3 + xy^2 - 4y^2`
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`(x^2 -xy + y^2).(x+y)`
` = x.( x^2 -xy + y^2) + y.(x^2 -xy + y^2)`
` = x^3 - x^2y + xy^2 + x^2y - xy^2 + y^3`
` = x^3 + y^3`
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