Elementary arithmetic order: when operating at the same level, it is calculated from left to right in turn; Two-stage operation, first multiply and then divide, then add and subtract.
When there are brackets, count the inside of brackets first, and then count the outside of brackets; When there are multiple brackets, count the brackets first, then the brackets inside, then the braces inside, and finally the brackets outside.
Combination:
Perhaps the most basic explanation of addition lies in combination: when two or more disjoint sets are combined into a single set, the number of objects in the single set is the sum of the number of objects in the original set.
This explanation is easy to imagine. It is also suitable for advanced mathematics; For a strict definition of its excitation, see the following natural numbers.
One possible solution is to consider a set of easily divisible objects, such as pies. Rods can not only form a set of rods, but also connect rods together, which explains another concept of addition: not adding rods, but adding the lengths of rods.