Hoặc
b) (B – C) + A.
b) (B – C) + A = (x2 + 3xy – y2) + (x2 – 2xy + y2)
= x2 + 3xy – y2 + x2 – 2xy + y2
= (x2 + x2) + (3xy – 2xy) + (y2 – y2)
= 2x2 + xy.