sentential logic derivations?
how do i prove the following is a theorem in SD
[(A -> B)->A]->A
... i started off by assuming [(A -> B)->A] then assume ~A to try to derive A in the end... but now i'm stuck :(
Suppose we dropped from SD the rule for vE, and adopted in its place the rule of Disjunctive Syllogism (DS), thus giving a modified system SD. Show that
you can derive the SD rule for vE in SD.
I have no idea how to even approach the second question.. any help would be greatly appreciated.. thanks! :D