$8\mathrm{.}8.$ Reconsider the portfolio selection example given in Section $8\mathrm{.}2.$ A fourth stock $($Stock $4)$ now has been found that gives a

good balance between expected return and risk. Using the same units as in Table $8\mathrm{.}2,$ its expected return is $17\%$ and its risk is

$18\%.$ Its joint risk per stock with Stocks $1,2,$ and $3$ is $-0\mathrm{.}015,-0\mathrm{.}025,$ and $0\mathrm{.}003,$ respectively.

a$.$ Still using a minimum acceptable expected return of $18\%,$ formulate the revised quadratic programming model in

algebraic form for this problem.

b$.$ Display and solve this model on a spreadsheet.

c$.$ Develop a revision of the parameter analysis report in Figure $8\mathrm{.}14$ for this revised problem.