The formula for the NPV is:
\(NPV=P_{AB}-(P_A+AmountPaid)\)
Note that \(P_{AB} = P_A+P_B+Synergies\)
Therefore
\(NPV=P_B+Synergies-AmountPaid)\)
which is how they are calculated. I do not see to which formula you refer to when you mention + PV of B
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