Derive hypothesis in Propositional logic I am learning to derive proofs of some sentences based on logical axiom schemes and inference rules. But there is a lot of unclear moments, like getting hypothesis. The one such example would be $A \Rightarrow B$ and I take $A$ and will use DT. Than if I have $A, B \mid- X$, i can take A, B. Is it possible from $A \vee B \Rightarrow X$ to take $A$ or $B$ ? And the same for $A \wedge B$. Can I take $A$ and $B$ as individual hypothesis ? 
Thanks for any description.  
 A: Let's say that your premises are $A$, $A \vee B \Rightarrow X$. 
Now you can use a rule of inference called addition and infer that $A \vee B$ is true. (This is not a hypothesis). 
Now using Modus Ponens, you can infer that $X$ is true.
The formal proof would look like this:
Premises
1)$\quad A$
2)$\quad A\vee B \Rightarrow X$
Proof
3)$\quad A \vee B\quad\quad\quad \quad\quad \text{Addition(1)}$
4)$\quad X\quad\quad\quad \quad\quad \text{Modus Ponens(2, 3)} $
You have proved that $X$ is true.

Now let's say that your premises are $A \wedge B$, $A \Rightarrow X$. You want to prove that $X$ is true.
Now you can use a rule of inference called Simplification (or Conjunction Elimination) and infer that $A$ is true. (This is also not a hypothesis). 
Using Modus Ponens, you can infer that $X$ is true.
The formal proof would look like this:
Premises
1)$\quad A \wedge B$
2)$\quad A \Rightarrow X$
Proof
3)$\quad A\quad\quad\quad \quad\quad \text{Simplification(1)}$
4)$\quad X\quad\quad\quad \quad\quad \text{Modus Ponens(2, 3)} $
A: Oky, maybe there is some kind of good learning materials. 
I have to understand the things I can do to prove something. 
The basic strategies if any. 
For example, I am looking to this one and just don't know the things i can do. 
(Xv!X)->(X->Z)v(Z->X). 
I usually try to create some basic hypothesis and try to find some good axiom scheme, but it works for 2 line proofs. Is it really so creative process to create the proof ?
