Distinguish between library functions (Pre defined) and user defined functions.

Library Function	User Defined Functions
LF(library functions) are Predefined functions.	UDF(user defined functions) are the function which r created by user as per his own requirements.
UDF are part of the program which compile runtime	LF are part of header file (such as MATH.h) which is called runtime.
In UDF the name of function id decided by user	in LF it is given by developers.
in UDF name of function can be changed any time	LF Name of function can't be changed.
Example : SIN, COS, Power	Example : fibo, mergeme