※ 引述《r833123 (持續前進 目標就會達到)》之銘言：
: http://i.imgur.com/2lyD5Dq.jpg
: 請問怎麼來的
=> -√[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

謝謝