12. Найдите значение выражения x³y+xy³ 5(x-y) 2(y-x) x²+y² при х=-3 и у=¹/₃

\[\begin{aligned} \frac{x^3y + xy^3}{2(y-x)} \cdot \frac{5(x-y)}{x^2 + y^2} &= \frac{xy(x^2 + y^2)}{2(y-x)} \cdot \frac{5(x-y)}{x^2 + y^2} \\ &= \frac{xy \cdot 5(x-y)}{2(y-x)} \\ &= \frac{5xy(x-y)}{-2(x-y)} \\ &= -\frac{5xy}{2} \end{aligned}\]

\[\begin{aligned} -\frac{5xy}{2} &= -\frac{5 \cdot (-3) \cdot (\frac{1}{3})}{2} \\ &= -\frac{-5}{2} \\ &= \frac{5}{2} \\ &= 2.5 \end{aligned}\]