Similar items: If two monomials contain the same letters and the indexes of the same letters are the same, they are called similar items. Such as 4y and 5y, 100ab and 14ab, 9c and 6c. In addition, all constant terms are similar terms (constant terms are also called digital factors).

Merge similar items: similar items in polynomials can be merged, which is called merging similar items.

After merging similar items, the coefficients of the obtained items are the sum of the coefficients of similar items before merging, and the letters and indexes remain unchanged. The letters are unchanged, and the coefficients are added and subtracted. The coefficients of similar items are added together, and the results are taken as coefficients, and the indexes of letters and letters remain unchanged.