FOILing (x - y)2 = (x - y)(x - y)
    F Take the product of the First terms x.x = x2
    O Take the product of the Outer terms x.(-y) = -xy
    I Take the product of the Inner terms (-y).x = -xy
    L Take the product of the Last terms (-y).(-y) = y2
    Then add them all up x2 - xy - xy + y2  =   x2 - 2xy + y2

Enter this as   x^2 - 2*x*y + y^2

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