SLOG for CSC165 week2

   It is the second week of this semester and I learnt a lot of new things in CSC165.
   I become more familiar with existential claims than the last week. And I am more skilled at evaluating quantified claims now. We learnt the universal and existential duality,which need us to draw Venn graphs with X and O and ?.I did a good job on this part ,which makes me confident.It is important to tell a sentence from a statement. All the statements are sentences but not all the sentences are statements. We can use quantifying to change an open sentence into a statement,which can be evaluated to true or false. I found it interesting that existential statement is not only about the interset of two sets are empty but also about one set is not a subset of the other one's counterset.We also learnt implication with the form of "if A,then B". A is the antecedent while B is the consequent. I also learnt a concept called contrapositive. Which occurs when we toggle and reverse an implication. For this part , it is a little complicated. Vacuous truth is also a little triky. It is always true except when antecedent is true and the consequent is false. It is hard to understand because it is different from the true in daily life. It works because there are no counterexamples. Then we are taught about different expressions of equivalence.
  I have gained a lot of knowledge this week and I would work hard to remember all of these things. See you next week!