А)(х/9z-z/x):(x-3z)/18zx= (x^2-9z^2)/9zx*18zx/(x-3z)= (x-3z)(x+3z)/9zx*18zx/(x-3z)= 2(x+3z);
б)(x/(x-1)+1):(1-3x^2/(1-x))= (x+x-1)/(x-1):(1-x-3x^2)/(1-x)= (2x-1)/(x-1)*(x-1)/(3x^2+x-1)= (2x-1)/(3x^2+х-1);
в) ((a+z)/z-2z/(z-a))*(z-a)/(a^2+z^2)= ((z+a)(z-a)-2z^2)/z(z-a)*(z-a)/(a^2+z^2)= (z^2-a^2-2z^2)/z(a^2+z^2)= (-(z^2+a^2))/z(z^2+a^2)=-1/z.