equivalence partition is a black box technique for writting 
test cases. 
 equivalence partition is process of reducing the hung
(infinite) set of possible into much smaller but still 
equally effective.
   In other word partition input condition which is tested, 
where each class representative(cover) a large set of other 
possible tests.
  In equivalence partion:-
   1) Range :- one valid         two invalid
   2) specific set:- one valid   one invalid.
   3) number:- one valid         two invalid