Đề bài

Tính:

a) $\frac{{{x^2} - 2xy}}{y}.\frac{{{y^2}}}{x}$;                                

b) $\frac{{{x^2} - 9{y^2}}}{{3x{y^2}}}.\frac{{xy}}{{x + 3y}}$;                           

c) $\frac{{1 - {x^2}}}{{2x + 4y}}.\frac{{{x^2} + 4xy + 4{y^2}}}{{3 - 3x}}$;         

d) $\frac{{{x^3} - {y^3}}}{{x + y}}.\frac{{{x^2} - {y^2}}}{{{x^2} + xy + {y^2}}}$