problem 45

$B △ A = (A \cup B) - (A \cap B)$ . By problem $44$ ,

\begin{matrix} P (B △ A) & = P (A \cup B) - P (A \cap B) = P (A) + P (B) - P (A \cap B) - P (A \cap B) = P (A) + P (B) - 2 P (A \cap B) \end{matrix}