# Thread: What the heck is "?" ???

1. ## What the heck is "?" ???

I'm going over my Discrete Structures assignment and an operator or something is used that I've never seen in the book or the notes. The questions is- Describe the following sets using the explicit method, with
Code:
` U = {x|x E N ^ 1<x<10}`
and then it gives sets A B C D and E explicitly, the questions im stuck on is - (C ? E) ? (A -(D ? B)) and B is complemented and D ? B is also complemented... what is that question mark, and what do I do with it??

2. I don't remember much from my Discrete Math class, but I found this:
A group of three people, each of which is a liar (lies all the time) or is a truar (tells the truth all the time) are talking. B says that C and D are the same type (both liars or both truars). Someone then asks D, ``Are B and C the same type?'' What does D answer?

Here's our solution. Let b stand for "B" is a truar", and similarly for c and d. The statement ``C and D are the same type'' is formalized as c == d. Since B said it, we have b == (c == d). D's answer can be formalized as b ? c, where ? is either == if D answers ``yes'' or /== if D answers ``no'', and we have to determine which it is. Since D answers, we write this as d == (b ? c).

We manipulate d == b ? c under the truth of b == c == d. In the manipulation,we omit all parentheses because == is associative and == and /== are mutually associative.

d == b ? c --Fact

= < Fact b == c == d >

b == c == b ? c
By inference rule Equanimity, the last line is a theorem. But the last line is a theorem only if ? is ==. Therefore, we conclude that D answered ``yes''.

3. actually the GA just emailed me back saying that there aren't any ? in the assignment and then I figured out that because im using OOo it read the word file wrong, too bad he emailed me back after the assignment was due.