  In this paper we derive three equations of motion for a supernova remnant
(SNR) in the framework of the thin layer approximation using the Pad\'e
approximant. The circumstellar medium is assumed to follow a density profile of
either an exponential type, a Gaussian type, or a Lane--Emden ($n=5$) type. The
three equations of motion are applied to four SNRs: Tycho, Cas A, Cygnus loop,
and SN~1006. The percentage error of the Pad\'e approximated solution is always
less than $10\%$. The theoretical decrease of the velocity over ten years for
SNRs is evaluated.
