the derivation of the efficient set when short sales are allowed and there is a riskless lending and borrowing rate is the simplest case we can consider. We know that the existence of a riskless lending and borrowing rate implies that there is single portfolio of risky assets that is preferred to all other portfolios. Furthermore, in return standard deviation space,this portfolio plots on the ray connecting the riskless asset and a risky portfolio that lies furthest in the counterclockwise direction