(aa+bb)(a+b) best problem though

fair

before i read problem: Your point is debatable, everyone takes a long time to prepare for problems. You can't trample on their efforts like that.

after i read problem: You're too conservative, let me teach you how to insult people.

OMG exactly! This problem be like: Some random subset of contestants with arbitrary ratings will solve this problem. With all respect to the author of course, not trying to hurt anyone but it's only fair to criticise their choice

what is (aa+bb)(a+b) problem?

I strongly disagree. It's perfectly OK to have a constructive problem in the problemset. I would've seen an issue if all problems were ad-hoc, but in this case it looks like you just didn't like the problem. But I believe nobody should blame author that their problem didn't match someone's taste.

Also, to not make 100500 comments, I'll answer to the Magician_Mathematician's comment here. It doesn't work like that for every problem there is a such number $$$B$$$ that participants with rating less than $$$B$$$ won't solve the problem while the participants with greater rating will. There is always some probabilistic distribution, and if variance was always low, it would probably be an incredibly dull technical contest with no cool ideas. I believe that “participants with random ratings solving a problem” is not that bad since it gives a chance to a lower rated participants to prove theirselves while also questions the higher rated participants' robustness and stability.

Also let's not forget that there were quite enough various problems for a two-hour contest. If an issue with a single problem D made one's participation poor might mean that either their strategy is not versatile enough or they're just too weak, not necessarily that the problemset is bad.