设 u=f(x^2+y^2+z^2)且f二次可微,求d^2u/dx^2

问题描述:

设 u=f(x^2+y^2+z^2)且f二次可微,求d^2u/dx^2

du/dx=2xf'
d^2u/dx^2=2f'+2xf''