>>854
P(A∪B)=P(A)+P(B)-P(A∩B)を繰り返し使う。

P(A∪B∪C)=P(A∪(B∪C))
=P(A)+P(B∪C)-P(A∩(B∪C))
=P(A)+P(B)+P(C)−P(B∩C)-P((A∩B)∪(A∩C))
=P(A)+P(B)+P(C)−P(B∩C)-P(A∩B)-P(A∩C)+P((A∩B)∩(A∩C))
=P(A)+P(B)+P(C)−P(B∩C)-P(A∩B)-P(A∩C)+P(A∩B∩C)