I need to apply Taylor series to linearize following equation
$$f_{dr}= Au'+ Bu + C u^2 +D$$
Here, $A$, $B$, $C$, $D$ are constants and $$u'=\frac{{\rm d}u}{{\rm d}t}, u=\frac{{\rm d}x}{{\rm d}t}$$
While applying Taylor series should I differentiate with respect to $t$ or $u$?