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3x+xy^2-x^2y-3y

a^2b-2b+ab^2-2a

2x+y+y^2-4x^2

a-3b+9b^2-a^2

3x+xy²-x²y-3y = (3x-3y) + (xy²-x²y) = 3 (x-y) + xy (y-x) = (x-y) (3-xy)

a²b-2b+ab²-2a = (a²b+ab²) - (2a+2b) = ab (a+b) - 2 (a+b) = (a+b) (ab-2)

2x+y+y²-4x² = (2x+y) + (y²-4x²) = (2x+y) + (y-2x) (y+2x) = (y+2x) (1+y-2x)

a-3b+9b²-a² = (a-3b) + (3b-a) (3b+a) = (a-3b) - (a-3b) (a+3b) = (a-3b) (1-a-3b)