1. Common factor extraction method: If a polynomial has the same constant, letter or polynomial, it can be extracted. This is very simple, just extract the same items.
2. Set formula method: If the multiplication formula is reversed, some polynomials can be decomposed into factors. This method is called formula method. Commonly used formulas include square difference formula, complete square formula and so on.
3. Cross multiplication: cross multiplication formula: first and last decomposition, cross multiplication and summation. There are generally two forms of cross multiplication: type I and type II.
4. Grouping decomposition method: Grouping decomposition is a concise method to solve equations. There are four or more items that can be grouped and decomposed. There are generally two forms of grouping decomposition: dichotomy and trichotomy.