The ran brings the question of the first tosfos on the masechta that what's the point of bedika medrabban if bittul is sufficient ? (Meaning acc to rashi what the point of Gezeiras Bedikah d'Rabanan) but then he goes to answer a whole diff question of that bittul and bedik both work mi'd'Oraisa.
But that wasn't the question the qeustion was why did the chachmim want u to make a bedika on top of the bittul and u see that he actually asks in the Chiddushei haran the question he asks here and then he Catullus give tosfos answer that u might come to eat then what is he asking in the first question
Mordechai yusupov, Queens, New York
1) We start with Rashi, bs'd. It may be that according to Rashi there in fact is no Gezeiras Bedikah d'Rabanan. Rashi writes that one checks the Chametz so that one will not transgess Bal Yera'eh and Bal Yimatzei. The simple reading of this is that Bedikah is mid'Oraisa. In fact, the Ran on the Rif writes (end of DH Ela) that his Shitah -- that mid'Oraisa either Bitul or Beidkah is sufficient -- is also the way of Rashi. So Rashi's opinion, according to the Ran, is that Bedikah is a Torah Mitzvah if one did not perform Bitul.
2) Now to the Ran's question in the name of Tosfos: If we look carefully, we see that the Ran does not ask what is the point of Bedikah mid'Rabanan if Bitul is sufficent? In fact, I think you will not find anywhere in the Ran on the Rif that he writes that Bedikah is only mid'Rabanan. Again, a close reading of the Ran on the Rif shows us that the question he reports in the name of Tosfos is not the same question that our Tosfos asks in the name of the Ri. The Ri indeed asks that since one anyway must do Bitul according to the Torah, why is it necessary to do Bedikah as well? However, the question the Ran asks in the name of Tosfos is different. He asks that since the Gemara on 4b states that mid'Oraisa Bitul alone is sufficient, and since the Gemara on 6b states that after one does Bedikah one anyway also has to do Bitul, how can Rashi say that by doing Bedikah one prevents himself from transgressing the Torah prohibitions? Since one also must do Bitul, and Bitul alone is sufficient, how can Bedikah work mid'Oraisa?
3) Again, it is worth noting that when the Ran cites the question of Tosfos, he does not say anything about Bedikah being mid'Rabanan, but rather the question was how can Bedikah be strong enough to exempt what it seems that only Bitul can exempt?
4) Now that we have seen that the Ran on the Rif never said that Bedikah is only mid'Rabanan, we can understand his answer to the first question in the name of Tosfos. The question was that since Bitul is sufficient mid'Oraisa, and since one must do Bitul, how can Bedikah take away the Torah prohibitions of Chametz? To that the Ran answers that either Bitul or Bedikah is sufficient mid'Oraisa.
5) Now to the Chidushei ha'Ran: He starts by asking the same question in the name of Tosfos that he asks in the Ran on the Rif, and basically gives the same answer. It is only then that he asks the question of the Ri from our version of Tosfos: what is the point of Bedikah if Bitul is sufficient? To that he gives the answer of the Ri that, with Bitul alone, one might come to eat it. The difference between the Ran on the Rif and the Chidushei ha'Ran is that the Ran on the Rif asks only the question of his Tosfos (not our Tosfos), while the Chidushei ha'Ran also asks the question of the Ri and gives the answer of the Ri.
Pesach Kasher v'Same'ach,
Dovid Bloom