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Thread: Probability rules

  1. #1
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    Probability rules

    If event A is a subset of event B, is it safe to say P(B-A) = P(B) - P(A)? In general, is it safe to say P(A+B) = P(A) + P(B), P(A*B) = P(A)*P(B), P(A/B) = P(A) / P(B), etc.?
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by seadog View Post
    If event A is a subset of event B, is it safe to say P(B-A) = P(B) - P(A)? In general, is it safe to say P(A+B) = P(A) + P(B), P(A*B) = P(A)*P(B), P(A/B) = P(A) / P(B), etc.?
    no.

    none of these are true in general. btw, what is P(A/B) supposed to mean?

    for that matter, what is P(B - A) and P(A + B) supposed to mean? are you using them to mean P(B and notA) and P(A u B) respectively? no one uses that notation.
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  3. #3
    MHF Contributor harish21's Avatar
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    Quote Originally Posted by seadog View Post
    If event A is a subset of event B, is it safe to say P(B-A) = P(B) - P(A)? In general, is it safe to say P(A+B) = P(A) + P(B), P(A*B) = P(A)*P(B), P(A/B) = P(A) / P(B), etc.?
    If event A is a subset of event B, then$\displaystyle P(A \cap B) = P(A)$.

    by $\displaystyle P(B-A), \mbox{do you mean} P(B)-P(A \cap B)$?? Then your answer would be yes.

    And what is P(A+B)?? Please clarify!
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  4. #4
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    Ok point taken. Anyways, here's a problem I'm kinda stumped on:
    "Let $\displaystyle A_1$, $\displaystyle A_2$,...,$\displaystyle A_n$ be a sequence of increasing events of a sample space, i.e. $\displaystyle A_i$ is a subset of $\displaystyle A_{i+1}$, $\displaystyle i \geq 1$.
    Let $\displaystyle B_1 = A_1$, $\displaystyle B_i = A_i - A_{i-1}$, $\displaystyle i \geq 2$. Note that $\displaystyle \bigcup_{i=1}^{\infty} A_i = \bigcup_{i=1}^{\infty} B_i$.

    a. Use the events $\displaystyle B_1$, $\displaystyle B_2$,... to prove that $\displaystyle \lim_{n \rightarrow \infty} P(A_n) = P(\lim_{n \rightarrow \infty} A_n)$. This proves that probability is a continuous function, because the increasing sequence of events {$\displaystyle A_n$, $\displaystyle n \geq 1$} is a convergence sequence."


    Does anyone have any idea how to prove lim P( ) = P[lim ( )] in general? I'll be happy if you can show me that; no need to help me completely for that whole part.
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