Question

（x²＋xy＋y²）²－4xy（x²＋y²）

Answer 1

(x²+xy+y²)²-4xy(x²+y²)

=(x²+y²)²+(xy)²+2xy(x²+y²)-4xy(x²+y²)

=(x²+y²)²+(xy)²-2xy(x²+y²)

=(x²+y²-xy)²