B = (2x +y)(4x^2 - 2xy + y^2) - (2x - y)(4x^2 + 2xy + y^2)
= (2x + y) [(2x)^2 - 2. x. y + y^2]- (2x - y) [(2x)^2 + 2. x. y + y^2]
= [(2x)^3 + y^3] - [(2x^3) - y^3]
= (2x)^3 + y^3 - (2x)^3 + y^3
= 2y^3
C = (5x - y)(25x^2 + 5xy + y^2) - (x - 2y)^3 - 3(2x + y)^3
= (5x)^3 - y^3 - (x^3 - 6x^2y + 12xy^2 - 8y^3)
= 3 . (8x^3 + 12x^2y + 6xy^2 + y^3)
= 125x^3 - y^3 - x^3 + 6x^2y - 12xy^2 + 8y^3 - 24x^3 - 36x^2y - 18xy^2 - 3y^3
= 100x^3 + 4y^3 - 30x^2 - 30xy^2
(2x - 3)^2 - (x - 3)^3 + (4x + 1) (16x^2 - 4x + 1)
= 8x^3 - 36x^2 + 54x - 27 - x^3 - 9x^2 + 27x - 27 + 4x(16x^2 - 4x + 1) + 1(16x^2 - 4x + 1)
= 8x^3 - 36x^2 + 54x - 27 - x^3 - 9x^2 + 27x - 27 + 64x^3 - 16x^2 + 4x + 16x^2- 4x + 1
= [(8x^3 - x^3) + (-36x^2 + 9x^2) + (54x - 27x) + (-27 + 27)] + [64x^3 + (-16x^2 + 16x^2) + (4x - 4x) + 1]
= 7x^3 - 27x^2 + 27x + 64x^3 + 1
= 71x^3 - 27x^2 + 27 + 1
(3x - y)^3 - (x + 2y)(x^2 - 2xy + 4y^2) + (3 + x)^2
= (27x^3 - 27x^2y + 9xy^2 - y^3) - (x^3 - 2xy^2 + 4xy^2 + 2yx^2 - 4y^2x + 8y^3) + (9 + 6x + x^2)
= (27x^3 - 27x^2y + 9xy^2 - y^3) -( x^3 + 8y^3) + (9 + 6x + x^2)
= 27x^3 - 27x^2y + 9xy^2 - y^3 - x^3 - 8y^3 + 9 + 6x + x^2
= (27x^3 - x^3) + (-27x^2y) + 9xy^2 + (-y^3 - 8y^3) + x^2 + 6x + 9
= 26x^3 - 27x^y + 9xy^2 - 9y^3 + x^2 + 6x + 9