x^2+y^2=x^2+y^2+2xy-2xy=(x+y)^2-2xy=a^2-2b(x-y)^2=x^2+y^2-2xy=x^2+y^2+2xy-4xy=(x+y)^2-4xy=a^2-4bx^2y+xy^2=xy(x+y)=ab
①a²－2b②a²－4b③ab
