6x²y*(x+y)/(x²-xy+y²)*(x+y) + 6xy*(x²-xy+y²)/(x²-xy+y²)*(x+y) - 12x³y/(x³+y³) = [заметим: x³+y³ = (x+y)(x²-xy+y²)] = (6x²y(x+y) + 6xy(x²-xy+y²) - 12x³y)/(x³+y³) =   6xy(x² + yx + x²- xy + y² - 2x²)/(x³+y³) = 6xy*y²/(x³+y³) = 6xy³/(x³+y³)

А = 6·x·y³, В = x³+y³