Question

9(x-y)²-12(y²-x²)+4(x+y)²

Answer 1

9(x-y)²-12(y²-x²)+4(x+y)²

＝[3(x-y)]²+2·[3(x-y)]·[2(x+y)]+[2(x+y)]²

＝[3(x-y)+2(x+y)]²

＝(5x-y)².