Thanks a lot for your reply. by instruction you give me the problem solved. but a more question: I write a lagrangian like what it mentioned in the file. "V" is a function of "q(t)" but is unknown. what change I have to do in the instruction to solve the Euler-Lagrange equation with respect to derivatives of "V"?    ELEOM2.mw 

@John Fredsted 

@Carl Love 

it is happened some mistake in writing that , it is now forth degree in v. what do you think now ?

thanks very much