Generally there are three methods to derive an approximation formula: 1) approching standard normal distribution by appropriate changing variable 2) using standardization variable for expansion 3) deriving approximation formula by direct method. This paper present correction terms having the form of C₁/v^M/2+C₂(n=1,2) with respect to X²^α distribution (v≤30),which minimize the error by EDA method and least square method.