Hii Fathimathul!! You sit in supplementary exam to pass those subjects which you were not able to do previously. You appear in this exam so that you become eligible to fill forms of those exams in which it is mandatory to have a passing certificate. Suppose you are giving supplensup of science paper and get paased then you are eligible to take course related to science subjects. Had not been this case then the whole concept of supplementary exam would be futile. Hope this helped. Good luck!!