B = (3x-y)^3 - (x+2y)(x^2 - 2xy + 4y^2) + (3-x)^2
⇔ B = 27x^3 - 27x^2y + 9xy^2 - y^3 - x^3 - 8y^3 + 9 - 6x + x^2
⇔ B = 26x^3 - 9y^3 - 27x^2y + 9xy^2 - 6x + x^2 + 9
Thay x = 1 , y = 2 ta được :
B = 26.1^3 - 9.2^3 - 27.1^2.2 + 9.1.2^2 - 6.1 + 1^2 + 9
⇔ B = 26 - 72 - 54 + 36 - 6 + 1 + 9
⇔ B = -60