Hey everyone,
Here's the problem:
Show that if and , then
I soooort of know how to start, it's supposed to be like,
We already have a derivation of from , so start with:
.
.
.
(k)
(k+1)
(k+2)
.
.
.
I have a hard time getting this naturally, because to me it feels like I should be able to assume in line (k+1) and then have ... but then, that's not right because it doesn't use Modus Ponens or any of the three axioms.
Any help or hints as to how I should be thinking?