5. (From a previous exam) Consider any linear program with an unrestricted variable z. When putting this problem into standard form, we perform the substitution x=rt-2- where + > 0 and 2 > 0. However, when doing Simplex, you will never have both + and r as basic variables simultaneously. Briefly explain why not. Hint: try making up a small example and see what goes wrong. Specifically, use the matrix representation to try to express the tableau