Hi, in Ch6, P.6 For a risky portfolio, after adding the risk-free portfolio, the new efficient frontier must be tangent to the old efficient frontier. Why there must be an efficient portfolio without risk-free asset in the new frontier? What if wherever risk-free asset is not included, it becomes inefficient? Same question for answer in Ch4, question 4.5(iv), why xc=0 must be one of the efficient point? Thanks a lot!
