Quote Originally Posted by Jhevon View Post
\begin{array}{lll} 1. & (P \wedge T) \implies (R \vee S) & \\<br /> 2. & Q \implies (U \wedge T) & \\<br /> 3. & U \implies P & \\<br /> 4. & \neg S & \backslash \therefore Q \implies R \\<br /> 5. & Q & \text{assumption} \\<br /> 6. & U \wedge T & \text{2,5 M.P.} \\<br /> 7. & U & \text{6, Simp.} \end{array}
<br /> \begin{array}{lll}<br /> 8. & T & \text{6, Comm., Simp.} \\<br /> 9. & P & \text{3,7 M.P.} \\<br /> 10. & P \wedge T & \text{8,9 Conj.} \\<br /> 11. & R \vee S & \text{1, 10 M.P.} \\<br /> 12. & R & \text{4,11 Comm., D.S.} \\<br /> \hline 13. & Q \implies R & <br /> <br /> \end{array}
By what law of logic do you justify line 13 of your proof?