combined std dev of corner portfolios whole, number, average, determine, ports

krause2

both CFAI & Schweser just do a straight weighted average of the std devs of two corner ports to come up with a combined number.

My understanding was that to determine combined std dev we need to do the whole calculation: square root of weightA^2*stddevA^2 + weightB^2*stddevB^2 + 2*Corr etc...

what am i missing here?

Windjammer

You not missing anything chief. Ignore the last part of the regular equation.