probability: Suppose (Omega) = {a,b,c}, P({a,b}) = 0.7, and
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pka
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\(\displaystyle \L
\begin{array}{rcl}
P(A \cup B) &= & P(A) + P(B) - P(A \cap B) \\
1 &= & P\left( {\left\{ {a,b,c} \right\}} \right) = P\left( {\left\{ {a,b} \right\}} \right) + P\left( {\left\{ {b,c} \right\}} \right) - P\left( {\left\{ {a,b} \right\} \cap \left\{ {b,c} \right\}} \right) \\
\end{array}\)
\begin{array}{rcl}
P(A \cup B) &= & P(A) + P(B) - P(A \cap B) \\
1 &= & P\left( {\left\{ {a,b,c} \right\}} \right) = P\left( {\left\{ {a,b} \right\}} \right) + P\left( {\left\{ {b,c} \right\}} \right) - P\left( {\left\{ {a,b} \right\} \cap \left\{ {b,c} \right\}} \right) \\
\end{array}\)