Method of substitution, a commonly used mathematical method in college entrance examination, is a variable substitution, which replaces another variable form with one variable form, thus simplifying the problem. The essence of substitution is transformation.
The undetermined coefficient method is a commonly used mathematical method in college entrance examination, and its essence is the idea of equation. This method unifies the unknown number and the known number in the equation relationship, so that the unknown number can be obtained by solving the equation (or equations).