  In this letter we show that the force-free Duffing-van der Pol oscillator is
completely integrable for a specific parametric choice. We derive a general
solution for this parametric choice. Further, we describe a procedure to
construct the transformation which removes the time dependent part from the
first integral and provide the general solution by quadrature. The procedure is
shown to have a wider applicability through additional examples. We also show
that through our method one can deduce linearizing transformations in a simple
and straightforward way and illustrate it with a specific example.
