# help with Propositional Logic??

i have to do an exercise for the university , i did that but i'm so sure that is right so:

• “The tumor is benign” A • “The tumor is metastatic” B • “The tumor has a good prognosis” C Express the following statements in Propositional Logic: 1. The tumor has a good prognosis only if it is either benign or not metastatic. 2. A tumor cannot be benign and metastatic at the same time. 3. If the tumor is not metastatic, it is either benign or has a good prognosis.

To my opinion the result should be:

1.A XOR ¬B ---> C

2.¬A AND B

3.¬B--->A XOR C

i read in Internet that i can convert either..or.. with the logical operation XOR but i'm not sure that my result is right because as second question is asked which of these formulas are equivalent, but i tried to do the true table and no one has the same result. Can everyone help me please?

• – Shaun Oct 21 '17 at 13:02
• Why are math examples always so morbid? – DanielV Oct 21 '17 at 16:37

For 1. "P only if Q" translates to $P \rightarrow Q$. The 'only if' works differently from a plain 'if': the 'if' expresses a sufficient condition, while the 'only if' expresses a necessary condition. So, "P only if Q" expresses that Q is necessary for P to be the case or, equivalently, that if Q is not the case, then P is not the case, i.e $\neg Q \rightarrow \neg P$ ... which by contraposition is equivalent to $P \rightarrow Q$
For 2. "Not both P and Q" says that it is not true that both P and Q are true, so that translates to $\neg (P \land Q)$