P ( B = β ) = ∑ α P ( A = α ) P ( B = β | A = α ) + 2 ∑ α 1 < α 2 cos θ α 1 α 2 P ( A = α 1 ) P ( B = β | A = α 1 ) P ( A = α 2 ) P ( B = β | a = α 2 ) {\displaystyle P(B=\beta )=\sum _{\alpha }P(A=\alpha )P(B=\beta |A=\alpha )+2\sum _{\alpha _{1}<\alpha _{2}}\cos \theta _{\alpha _{1}\alpha _{2}}{\sqrt {P(A=\alpha _{1})P(B=\beta |A=\alpha _{1})}}P(A=\alpha _{2})P(B=\beta |a=\alpha _{2})}
