In terms of the undetermination method,this article discusses and provides the sufficient and necessary conditions of the transformation from variable coefficient equations set to solvable one with some alterations.

As a result,the proof is given to the theorem that"the bearings only locating and tracking system remains unsolvable prior to an observer maneuver.

Finally, its solvable cases are given.

Establishment of oilfield development planning models their solution;

The angular dimmension chain and new method for solution in mechanical design;

Establishment and solution to spatial vibration equation of train-track(bridge) time-varying system;

Concealed Conditions and the Solving of Questions in Mechanics;

Analysis and Solving about Derivative Problems Arisen in Reform of Fee to Tax in Country;

By using interval trial programming technique, this paper introduces a practical method for obtaining the best solution from multi solution models such as hyperbolic and Weng model and the skills of fast solving.