※ 引述《r833123 (持续前进 目标就会达到)》之铭言：
: http://i.imgur.com/2lyD5Dq.jpg
: 请问怎么来的
=> -√[x^2 + y^2] [xdx + ydy] = 0
=> -1/2 √[x^2 + y^2] d[x^2 + y^2] = 0
=> -(1/2)(2/3)d[(x^2 + y^2)^(3/2)] = 0

谢谢