Why is a portfolio's beta just the weighted average of its stocks' betas, and what costs of capital does that give Microsoft and Gillette?
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Portfolio beta
Derive it by dividing the covariance identity by , contrast it with portfolio variance and its cross products, and price a project from comparable stocks' betas, such as oil drilling.
A new stock's covariance with the market
Write the count of variances against covariances, state that an investor holding M cares only about a new stock's covariance with M, and record the effect of and .
Costs of capital for Microsoft and Gillette
Record the betas 1.49 and 0.81 with the downturn smell test, then compute 13.94% and 9.86% from a 5% risk-free rate and a 6% market premium.