2楼: Originally posted by hank612 at 2014-10-23 13:26:51
let dx/dt =p as a new variable, then \frac{d^2x}{dt^2}=p\cdot \frac{dp}{dx}
This gives  v^2=-2\frac{g}{l}(\frac{x^2}{2}-\frac{x^4}{24})+C_1

Then remaining task is an integral problem, although  ...