[PERFORM] how explain works

[Radhya sahal]

Dear ,all
plz could any one help me !!!
how explian works as math equations to estimate cost with  constatn query 
parameters
such as cpu_tuple cost ,random page cost ...etc
 i want maths  expression  in order to know how these parameters will effect in 
cost ???
please any one can help me ??
 
 
Regards
Radhya...