3(x+y) в квадрате-6xy=
3(x^2+2xy+y^2)-6xy=3x^2+6xy+3y^2-6xy=3x^2+3y^2=3(x^2+y^2)
3(x^2+2xy+y^2)-6xy=3x^2+6xy+3y^2-6xy=3x^2+3y^2