IAA loss model question

Missing something stupid here, and probably superflous but:





Cov (XiBi, XiBi) =? Var (XiBi) =



Var(Bi)*E(Xi)^2 + Var(Xi) * E[Bi)^2 =



b * E(Xi)^2 + Var(Xi) * (1)^2 = b * E[Xi)^2 + Var(Xi)





IAA has this as:



b * E[Xi)^2 + Var(Xi) * (1+b)





where does the 1+b come from?



Thanks!

IAA loss model question