542 sats \ 9 replies \ @Scroogey 25 Oct \ on: [Daily puzzle] is this number rational science
We can use the trigonometric addition formula
in an inductive proof by contradiction:
  • Assumption: is rational.
  • Step: if is rational then must also be rational, because the above formula is the ratio of two rational numbers.
  • But which is irrational, hence the assumption must be wrong.
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11 sats \ 2 replies \ @south_korea_ln OP 25 Oct
Nice job. This was supposedly an entrance exam question for the University of Kyoto in 2006.
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160 sats \ 1 reply \ @Rothbardian_fanatic 25 Oct
I understand the Kyodai and Todai exams are literal nightmares! They have people going to special exam schools for several years on end to get the score high enough to be accepted.
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0 sats \ 0 replies \ @south_korea_ln OP 26 Oct
Yeah looks pretty extreme. You really need to be trained on this to be able to solve this kind of problem with tight time constraints.
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1 sat \ 5 replies \ @SimpleStacker 25 Oct
Nice. Are you a mathematician, @Scroogey?
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0 sats \ 4 replies \ @Scroogey 25 Oct
No, I enjoyed college level math thirty-plus years ago, but was more interested in computer programming than pure math.
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1 sat \ 3 replies \ @SimpleStacker 25 Oct
Pulling out an inductive proof by contradiction, along with an obscure (to me) formula, is impressive for someone who doesn't do proofs on a regular basis :)
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0 sats \ 2 replies \ @Scroogey 25 Oct
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.
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1 sat \ 1 reply \ @SimpleStacker 25 Oct
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.
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22 sats \ 0 replies \ @south_korea_ln OP 26 Oct
Yeah, Reductio ad Absurdum were also my favorites.
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