Quote by sweet springs
= 2a0 dr/dt e + 2a1 (dr/dt)^2 e + 2a2 (dr/dt)^3 e + ...
Only the first term is the time derivative of the function, say f=2a0re.
Regards.

I see. df(x,t)/dt = df/dx v + df/dt is a linear function in v so the higher order terms are eliminated.
Thank you!
evoluciona