Question 1155652
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1.  P v Q   
2.  P → (T → S),  
3.  P → T,  
4.  S ↔ Q          ├  S

               |5.  ~S                  Assumption for Indirect Proof
               |6.  (S → Q) & (Q → S)     4,  Material Equivalence
               |7.  (Q → S) & (S → Q)     6,  Commutation
               |8.  Q → S                 7,  Simplification
               |9.  ~Q                  8,5,  Modus Tollens
               |10. Q v P                 1,  Commutation
               |11. P                  10,9,  Disjunctive Syllogism
               |12. T                   3,11, Modus Ponens    
               |13. T → S               2,11, Modus Ponens
               |14. S                  13,12, Modus Ponens
               |15. ~S & S              5,14, Conjunction
16. S                                 Lines 5-15 Indirect Proof

Edwin</pre>