[tex]\frac{(x^{2}-y^{2}).(y^{2}-z^{2}).(z^{2}-x^{2})}{(x-y)^{3}.(y-z)^{3}.(z-a)^{3}}\\\\ =\frac{(x-y).(x+y).(y-z).(y+z).(z-x).(z+x)}{(x-y).(x^{2}+xy+y^{2}).(y-z).(y^{2}+yz+z^{2}).(z-x).(z^{2}+za+a^{2})}\\\\ =\frac{(x+y).(y+z).(z+x)}{(x^{2}+xy+y^{2}).(y^{2}+yz+z^{2}).(z^{2}+zx+x^{2})}[/tex]