what if you listen to the siuyum but you do not go to the seudah or partake of the seudah-did you fulfiill the obligation and not have to fast

Shalom, The reason first born do not generally fast nowadays on the eve of Passover is because they participate in a mitzvah meal. Classic mitzvah meals include those held for a Brit or Pidyon HaBen. As it is not always possible to have a brit every Passover eve, the widespread custom is to partake of a siyum meal. What exempts the first born is the participation in such a meal, since the eating there is of a mitzvah status and therefore it overides the fast. So one does have to partake of the meal, and not just hear the siyum. Blessings.