第1个回答 2016-04-21
z = f[√(x^2+y^2)]
∂z/∂x = [x/√(x^2+y^2)]f'
∂^2z/∂x^2 = [y^2/(x^2+y^2)^(3/2)]f' + [x^2/(x^2+y^2)] f''
同理 ∂^2z/∂y^2 = [x^2/(x^2+y^2)^(3/2)]f' + [y^2/(x^2+y^2)] f''
∂^2z/∂x^2 + ∂^2z/∂y^2 = f'/√(x^2+y^2) + f'' = 0
即 f'’(u) + f'/u = 0本回答被提问者和网友采纳