Pythagoras theorem prove

“Secret name” says:

My philosophy teacher, who I do not know why he thinks he can do math, tried to prove Pythagoras theorem and drew that picture on the board. Then he said “It proves Pythagoras theorem because 3^2 + 4^2 = 5^2, as shown in the picture”. Then I said I had not understood why it proves Pythagoras theorem.

He asked to the class if someone else had not understood and they did not said anything. Then he said (not with those words) I am stupid and the prove was trivial.

Thanks to “secret name” for this submission.

[[Editor’s note: Unfortunately this happens all too often when a teacher does not know an answer. But don’t let it be a reason to discourage you from math!]]

** Note: Some posts on Math-Fail are user-submitted and NOT verified by the admin of the site before publication. If you find this post to be distasteful, non-math related, ?or something worse?, then definitely leave a comment letting me know. Thanks very much! Mike **

1 Star2 Stars3 Stars4 Stars5 Stars (4.92 from 24 votes)


  1. This reminds me of my proof that if you flip a coin it will come up heads.

    Thumb up 1 Thumb down 0

  2. Otto Bob Otto

    Sigh, I sure hope this wasn’t the whole proof. Proving something for one case does just that, it proves it for that single case.

    Wanna see? Claim: 2x = x^2. Proof: let x = 2. Both sides simplify to 4. QED.

    Thumb up 4 Thumb down 0

  3. This reminds me why science > philosophy

    Thumb up 3 Thumb down 1

  4. Well, either this is false, or your teacher is stupid, because someone who studies philosophy, while not being able to do math, learns pretty well the rules of logic and mathematical proofs, so no one would say that.

    Thumb up 0 Thumb down 0