Logic 101 (#38): Working Backward (Killer Proof Strategy #2)

If you aren't sure how to prove what you need prove, work backward. Figure out what is sufficient to prove the conclusion, then prove those things. If you can, some minor editing will give you a complete and valid proof.

[PunmasterSTP]

Killer proof? More like "raise the roof", because this video was great!

[pokemonomekop]

just wanted to let you know that I really appreciate the amount of work you've put behind this entire channel
I've finished your game theory 101 series and am working on civil wars and logic 101
I'm not in college yet so I haven't taken these courses, but it's super interesting, so I'm probably going to end up taking it then. Hopefully I don't die in the math because I love this kind of stuff
please keep it up :)

[digitalmagiciandev]

this series saved me headache, but THIS video saved my grade

[samidib]

william thank u so much you are able to explain this far better than my professor ever could. You are amazing and so happy your channel is still alive and well! I am going to pass this class just because of u, these videos on logic have given me hope and bravery to not only learn and see what I'm doing but actually solve the problems without being confused I am now thinking and realizing the mistakes I make> thank you so much!!!

[GustavoMonaco]

William! Thank you so much for this! I've been having difficulties on trying to grasp the proof methods. I just completed your 3 videos and I was able to do them on my own! I hope you can continue your good work here and bring more Killer Proofs! Actually, if you happen to have those killer proof methods somewhere, could you share them while you get the rest of the videos recorded? 

Keep up the great work! Thank you again!

[hz]

thank you so much man thank you so so much !!!!

[danieldude]

I don't agree with 2:59 ... if P and ~Q is false then line 3 is still true... but you said that because ~R is true then we need to prove P and ~Q is true even tho its not necessary. am I wrong thinking that you are wrong?

[PMartinez]

This would have been so useful last quarter lol.

[ardabaser]

If p and not q then not r. You need to prove that not r is true because without it you can't prove s is true. Nothing wrong so far. However, not r being true does not prove that p and not q is true. You see, p and not q can be either true or false since 1=>1 and 0=>1 are both true. Although I believe the proof is true overall, I do not agree with the part where you claim that not r being true proves that p and not q is true.

[miltonpalaguachi]

I don't understand how to get from line 4 to 5 (P v Q) AND ~Q <=> ~Q

[TheMorhaGroup]

Did you not prove, P ^ -Q, by using it to prove, P v Q. I just don't see how that works? I was able to solve it as well, I will admit it took me quite a while, but my method was far more rigorous than this