因为偏z/偏x=[1/(x^2+y^2)^(1/2)] - (x^2)(x^2+y^2)^(-3/2)偏z/偏y=-xy(x^2+y^2)^(-3/2)所以全微分dz=[[1/(x^2+y^2)^(1/2)] - (x^2)(x^2+y^2)^(-3/2)]dx+[-xy(x^2+y^2)^(-3/2)]dy
