What makes the tangency line's slope, the Sharpe ratio, the one number a mean-variance investor maximizes, and why can a new stock never lower it?
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Lending, full investment and borrowing along the tangency line
Sketch the line from through the tangency portfolio, label the lending, fully invested and leveraged regions, and give the weight in T-bills for one point in each region.
Limits of the picture: kink, shorting, static snapshot
Record that unequal borrowing and lending rates put a kink in the line, draw the reflected down-and-right line from shorting the tangency portfolio, and note that the analysis is a one-period snapshot.
The Sharpe ratio
Write the formula as rise over run of the line from , compute it for two numerical portfolios, and reproduce the geometric argument that the tangency portfolio has the highest Sharpe ratio.
Adding a security relaxes a zero-weight constraint
Write the 20-stock portfolio as a 21-weight portfolio with , state that the new frontier is never worse, and record that any correlation below suffices to bend it left.