Specific examples were cited to analyze the mistakes and causes in the use of integration by substitution, and the general method was pointed out to avoid such kinds of mistakes in operation.

The integration by second substitution is an important method of calculating indefinite integral, it has a certain application, it usually applies to calculate some integrals of irrational function.

The steps of design by reuse based on similarity unit substitution and system integration design method based on units are discussed.