Thu gọn biểu thức:

a) \(\frac{{27}}{{15}}{x^3}y{z^5}:\frac{9}{5}x{z^2}\)

\( = {x^2}y{z^3}.\)

b) \(3x\left( {x - y} \right) - y\left( {y - 3x} \right)\)

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

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

c) \(\left( {x - y} \right)\left( {{x^2} + {y^2}} \right) - \left( {{x^4}y - x{y^4}} \right):xy\)

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

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

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