Although written in the form of O (x 3), two O (x 3) can be different.

For example, the o (x 3) in the first row can be x 4, and the o (x 3) in the second row is (6x) 5/5! + ...

The subtraction between them is obviously not 0. Although it does not affect the calculation results here, it must also be expressed by O (x 3).

A more general conclusion is:

o(x) o(x)=o(x)