Strong duality theorem

Consider the following primal linear-programming problem and its dual problem:

Some authors call the following basic duality theorem the strong duality theorem: If either the primal or the dual has a finite optimal solution, then the other problem has a finite optimal solution, and the optimal values of their objective functions are equal, that is, minimum cv ^T xv = maximum bv ^T yv. They give the name weak duality theorem to the theorem: If xv is a feasible solution to the primal problem and yv is a feasible solutuion to the dual problem, then bv ^T yv ≤ cv ^T xv.