In the context of AE analysis, design economics enters into play when two or more cost components making up the equivalent cost are affected differently by the same design variable.  When a cost component varies directly with respect to a design variable and some other cost component varies inversely with respect to the same design variable, then the cost components can be said to be affected differently by the same design variable.  In Example 8.10, the design variable is A, the cross-sectional area of the conductor, and the cost components are the capital recovery cost and the annual operating cost.  The design variable A affects these two cost components differently and thus the search for the optimal value of the design variable will have to go through the economics of optimization which is explained in the derivation leading to equations 8.5 and 8.6.