- #1
hassman
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Homework Statement
Prove that [itex]P(A \cap B)≥1-P(\bar{A})-P(\bar{B})[/itex]
for all [itex] A, B \subseteq S[/itex]using only these axioms:
1) [itex]0 \leq P(A) \leq 1[/itex], for any event [itex]A \subseteq S[/itex]
2) [itex]P(S) = 1[/itex]
3) [itex] P(A \cup B) = P(A) + P(B)[/itex] if and only if [itex] P(A \cap B) = 0[/itex]
Homework Equations
None.
The Attempt at a Solution
My proof:
First rearrange the shizzle:
[itex]P(\bar{A}) + P(A \cap B) + P(\bar{B}) \geq 1[/itex]
Now using the fact that the first two terms are disjoint, use axiom 3 to obtain:
[itex]P(\bar{A} \cup (A \cap B)) + P(\bar{B}) \geq 1[/itex]
Next note that the first term is equals to P(S), hence we get:
[itex]P(S) + P(\bar{B}) \geq 1 [/itex]
which holds for all [itex] B \subseteq S[/itex], because [itex]P(S) = 1[/itex] and [itex] P(\bar{B}) \geq 0 [/itex] for all B.
Is this proof correct?