The former differential coefficient of a function f ( x ) is now called its derivative df ( x ) \/ dx . It is the ( not necessarily constant ) multiplicative factor or coefficient of the differential dx in the differential df ( x ) . In mathematics , the differential coefficient of a function f ( x ) is what is now called its derivative df ( x ) \/ dx , the ( not necessarily constant ) multiplicative factor or coefficient of the differential dx in the differential df ( x ) .