Thực hiện phép tính: (a) (−1/3xy)(−3x^2y+2xy−9x).

a) \(\left( { - \frac{1}{3}xy} \right)\left( { - 3{x^2}y + 2xy - 9x} \right)\)

\( = - 3{x^2}y \cdot \left( { - \frac{1}{3}xy} \right) + 2xy \cdot \left( { - \frac{1}{3}xy} \right) - 9x \cdot \left( { - \frac{1}{3}xy} \right)\)

\( = {x^3}{y^2} - \frac{2}{3}{x^2}{y^2} + 3{x^2}y.\)

b) \(\left( {{x^3}{y^3} - 5{x^2}{y^5} + \frac{1}{{10}}x{y^2}} \right):\left( {x{y^2}} \right)\)

\( = {x^3}{y^3}:\left( {x{y^2}} \right) - 5{x^2}{y^5}:\left( {x{y^2}} \right) + \frac{1}{{10}}x{y^2}:\left( {x{y^2}} \right)\)

\( = {x^2}y - 5x{y^3} + \frac{1}{{10}}.\)

c) \(\left( {2x - y} \right)\left( {{x^2} - xy + 3{y^2}} \right)\)

\( = 2x\left( {{x^2} - xy + 3{y^2}} \right) - y\left( {{x^2} - xy + 3{y^2}} \right)\)

\( = 2{x^3} - 2{x^2}y + 6x{y^2} - {x^2}y + x{y^2} - 3{y^3}\)

\( = 2{x^3} + \left( { - 2{x^2}y - {x^2}y} \right) + \left( {6x{y^2} + x{y^2}} \right) - 3{y^3}\)

\( = 2{x^3} - 3{x^2}y + 7x{y^2} - 3{y^3}.\)