11（2） z = √(x^2+y^2), dz = ( xdx+ydy)/ √(x^2+y^2)⊿z = ( x0⊿x+y0⊿y)/ √(x0^2+y0^2) = [4*0.05+3(-0.07)]/√(4^2+3^2) = -0.01/5 = -0.002z = z0+⊿z = 5 - 0.002 = 4.998
