# Math Help - Discrete set theory.

1. ## Discrete set theory.

this is some kinda of discrete set theory I want help with, heres the question and what I have so far, can someone explain what P(A) is, P(~A^B) and (P^~B)
also what are the independent variables.

I've included the questions in a pic, and i've written my work the best I could.

So
P(A^B) :: 0.3
P(A|B) :: 0.5 :: P(A) / P(A^B) , So, P(A) :: 0.5 * 0.3 :: 0.15
Now, P(~A^~B) :: 0.1 ,or , P(A v B) :: 1 -0.1 :: 0.9 :: P(A) +P(B) - P(A^B) , which gives P(B) is greater than 1.0 , this can't happen!

Originally Posted by rcmango
this is some kinda of discrete set theory I want help with, heres the question and what I have so far, can someone explain what P(A) is, P(~A^B) and (P^~B)
also what are the independent variables.
I've included the questions in a pic, and i've written my work the best I could.
So
P(A^B) :: 0.3
P(A|B) :: 0.5 :: P(A) / P(A^B) , So, P(A) :: 0.5 * 0.3 :: 0.15
Now, P(~A^~B) :: 0.1 ,or , P(A v B) :: 1 -0.1 :: 0.9 :: P(A) +P(B) - P(A^B) , which gives P(B) is greater than 1.0 , this can't happen
None of this makes any sense.
I take it that this is about probability?
What does "::" mean?
What do you know about the rules of a probability measure?

Originally Posted by Plato
None of this makes any sense.
I take it that this is about probability?
What does "::" mean?
What do you know about the rules of a probability measure?
:: means equals =
..hmm, if it didn't make any sense i would prefer to wait for someone who can make some sense out it, thanks

4. ## Re: Discrete set theory.

The following line
Originally Posted by rcmango
P(A|B) :: 0.5 :: P(A) / P(A^B)
looks suspicious because $P(A|B) = \frac{P(A \cap B)}{P(B)}$.

5. ## Re: Discrete set theory.

This is probability, not set theory. You might conflate the two because P(A) in set theory means "the power set of the set A", whereas here it means "the (prior) probability of event A". You might get more help if you put this in the undergrad probability forum.

6. ## Re: Discrete set theory.

Originally Posted by Annatala
This is probability, not set theory. You might conflate the two because P(A) in set theory means "the power set of the set A", whereas here it means "the (prior) probability of event A". You might get more help if you put this in the undergrad probability forum.
Thank you for responding to this post.
However, we do not encourage double postings.
The objection to the OP is that it is really nonstandard notation.
If posters do not know how to use LaTeX at least they can use standard mathematical notation.

Frankly, I see no reason to help someone who makes no effort to make his/hers question question readable.

7. ## Re: Discrete set theory.

No problem. I didn't know if there was a method for sending a post to another forum, or quashing it in one so it can be spawned somewhere more appropriate. I don't expect as many people who do probability on a regular basis would be here rather than there, though the question is simple enough that it's not that relevant.