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?