Let ABC and DEF be triangles with

    ∠ABC = ∠DEF > 90°,
    |BC| = |EF|, and
    |CA| = |FD|.

Prove that ΔABC ≅ ΔDEF with a direct geometric proof.

(In reply to re: First part of solution by Bractals)

I realized after I posted the solution that I could have just used the pythagorean theorem.  Well, I guess I accidentally proved the pythagorean theorem instead.

I've never heard of the Steiner-Lehmus theorem.  My initial thought is that "direct geometric proof" is not really a well-defined category.  Is there some way to divide elementary logical inferences into "direct" and "indirect" categories?  Not sure.

Posted by Tristan on 2013-10-05 01:03:52