cho a=x^3y-xy^3+y^3z-yz^3+z^3x/x^2y-xy^2+y^2z-yz^2+z^2x-zx^2 a) với giá trị nào của x,y,z thì A có nghĩa b) tính giá trị của A khi x=-1/2, y=5/2,z=8

${\displaystyle \frac{x^{3} y-xy^{3} +y^{3} z-yz^{3} +z^{3} x}{x^{2} y-xy^{2} +y^{2} z-yz^{2} +z^{2} x-zx^{2}}}$ 
Thay ${\displaystyle x=\frac{-1}{2} ;y=\frac{5}{2} ;z=8}$ ta có
${\displaystyle \frac{\left({\displaystyle \frac{-1}{2}}\right)^{3} .{\displaystyle \frac{5}{2}} -({\displaystyle \frac{-1}{2}} ).{\displaystyle \frac{5}{2}}^{3} +{\displaystyle \frac{5}{2}}^{3} .8-{\displaystyle \frac{5}{2}} .8^{3} +8^{3} ({\displaystyle \frac{-1}{2}} )}{\left({\displaystyle \frac{-1}{2}}\right)^{2} .{\displaystyle \frac{5}{2}} -\left({\displaystyle \frac{-1}{2}}\right) .{\displaystyle \frac{5}{2}}^{2} +{\displaystyle \frac{5}{2}}^{2} .8-{\displaystyle \frac{5}{2}} .8^{2} +8^{2} .({\displaystyle \frac{-1}{2}} )-8({\displaystyle \frac{-1}{2}} )^{2}} =\frac{5614}{561}}$