（1）5x^-1y^4/4x^2y=5/x*y^4/4x^2y=5/4*x^(1-2)y^(4-2)=5/4*x^-1y^2(2)(x^2-9/x^2-6x+9)^-3=[(x+3)(x-3)/(x-3)^2]^-3=[(x+3)/(x-3)]^-3=1/[(x+3)/(x-3)]^3=1/[(x+3)^3/(x-3)^3]=(x-3)^3/(x+3)^3