[Lesson] Proofwriting

[d&q]

Here are two more proofwriting laws:





You can put this under the Contrapositive section:

[Sidoh]

I thought these might be helpful as well:

[Ender]

You can put this under the Contrapositive section:

I believe your definition of the contrapositive is wrong. If P => Q then the contrapositive is not Q => not P, instead of not P => not Q.

For instance:

(1) If you live in Massachusetts then you live in the USA (P => Q) -- correct
(2) If you do not live in Massachusetts then you do not live in the USA (not P => not Q) -- incorrect
(3) If you do not live in the USA then you do not live in Massachusetts (not Q => not P) -- correct

Statements (1) and (2) do not have to be either both true or both false, as shown above. Statements (1) and (3) do have to be either both true or both false, which is what I think you meant by the Law of the Contrapositive.

[d&q]

? I haven't the slightest idea what you mean.