For the mixed operation of addition, subtraction, multiplication and division of rational numbers, if there is no bracket to indicate what operation to do first, it will be performed in the order of "multiplication and division first, then addition and subtraction", and if it is the same level operation, it will be calculated from left to right.

In the formula without brackets, if there is only addition, subtraction, multiplication and division, it should be calculated from left to right.

In the formula without brackets, if there are both multiplication and division and addition and subtraction, the multiplication and division are calculated first, and then the addition and subtraction are calculated.

For formulas with brackets, count the ones in brackets first, and then the ones in brackets.

Extended data 1. Generally speaking, the calculation order of the four operations is: if there are brackets, calculate the ones in brackets first; Only the same level of operation, from left to right; Contains two levels of operation, multiplication and division first and then addition and subtraction.

2. Because some calculation problems have their own characteristics, using the algorithm can make the calculation process simple and not easy to make mistakes.

Additive commutative law: a+b=b+a?

Multiplicative commutative law: a×b=b×a

Additive associative law: (a+b)+c=a+(b+c)

Law of multiplicative association: (a×b)×c=a×(b×c)