Thu gọn biểu thức:

a) \(\left( { - 9{x^2}{y^3} + 6{x^3}{y^2} - 4x{y^2}} \right):3x{y^2}\)

\( =  - 9{x^2}{y^3}:3x{y^2} + 6{x^3}{y^2}:3x{y^2} - 4x{y^2}:3x{y^2}\)

\( =  - 3xy + 2{x^2} - \frac{4}{3}.\)

b) \(\frac{1}{2}xy\left( {{x^5} - {y^3}} \right) - {x^2}y\left( {\frac{1}{4}{x^4} - {y^3}} \right)\)

\( = \frac{1}{2}xy \cdot {x^5} - \frac{1}{2}xy \cdot {y^3} - {x^2}y \cdot \frac{1}{4}{x^4} + {x^2}y \cdot {y^3}\)

\( = \frac{1}{2}{x^6}y - \frac{1}{2}x{y^4} - \frac{1}{4}{x^6}y + {x^2}{y^4}\)

\( = \left( {\frac{1}{2}{x^6}y - \frac{1}{4}{x^6}y} \right) - \frac{1}{2}x{y^4} + {x^2}{y^4}\)

\[ = \frac{1}{4}{x^6}y - \frac{1}{2}x{y^4} + {x^2}{y^4}.\]