What is the undetermined coefficient method Ditto (detailed)

What is the undetermined coefficient method Ditto (detailed)

In this paper, a polynomial is expressed as a new form with undetermined coefficients, and an identity is obtained. Then, according to the nature of the identity, the equation or system of equations that the coefficients should satisfy is obtained. Then, by solving the equation or system of equations, the undetermined coefficients can be obtained, or the relations that some coefficients satisfy can be found