Article (9) builds a model of Associated Optimization on project investment and brings out theorems (3) and (6) on mutual equivalence. The article proves that Yang's theorem cannot hold water when theorems (3) and (6) are in the interval in which they cannot be judged whether good or not. With the demonstrative analysis in article (9)， it proves that pure maximization of economic benefit is the scientific rule of how to select the better in two mutually exclusive schemes.