Yeah, _Reductio ad Absurdum_ were also my favorites.

south_korea_ln

Proof by contradiction was my favorite method in college.  I think it's easier because you know where to start and you just move forward.  So even to prove that things are true, I'd just assume the converse and show a contradiction.

Because a non-contradiction based proof seems harder.  You know where you want to go, but you don't know where to start.

SimpleStacker

science

[Daily puzzle] is this number rational

I remember such proofs were considered less elegant or even rejected by some mathematicians in earlier times, but don't know the historic context anymore. 