b) (x + 2y)(x^2 – 2xy + 4y^2) + (2x – y)(4x^2 + 2xy + y^2).
b) (x + 2y)(x2 – 2xy + 4y2) + (2x – y)(4x2 + 2xy + y2).
b) (x + 2y)(x2 – 2xy + 4y2) + (2x – y)(4x2 + 2xy + y2)
= (x + 2y)[x2 – x . 2y + (2y)2] + (2x – y)[(2x)2 + 2x . y + y2]
= (x + 2y)[x2 – x . 2y + (2y)2] + (2x – y)[(2x)2 + 2x . y + y2]
= x3 + (2y)3 + x3 – (2y)3
= x3 + x3 = 2x3.