Question 1149388
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Needing help with line 4 is the entire problem :-)  If this helps, take time to send 'thanks'... it is much appreciated:

1. P --> S     Premise
2. P v Q       Premise
3. Q --> R     Premise
// Show S v R

4. ~P v S      1, Material Implication (MI)
5. S v Q       2,4 Resolution (RES)
6. ~Q v R      3, MI
7. R v S       5,6 RES   
*** DONE ***

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Alt Proof:
4. R v S       1,2,3 Constructive Dilemma (CD)
*** DONE ***

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Alt Proof #2 (Conditional proof):
4.::  ~P       Conditional proof (CP) assumption #1
5.::   Q       4,2 (RES)
6.::   R       5,3 Modus Ponens (MP)  // we've shown R true if P false
7.::   P       CP assumption #2
8.::   S       7,1 MP    // we've shown S true if P true
9.::   S v R   4-8  Proof by Cases (PBC)
10. S v R      4-9 CP
*** DONE ***

In CPs, you are applying hypotheticals/scenarios and seeing what logically 
follows; any interesting/useful conclusions can be extracted but the states of the variables do not change (for instance, one can NOT say R is true from line 6::, because that only followed from the ~P assumption on line 4::).  I like to think of CPs as a way to see how the logic flows, without permenantly changing anything.