In the textbook of the first grade of primary school, the unspoken rule is this: subtraction is used for all dotted lines. Anything you don't use can be added or subtracted. Only addition and subtraction or only multiplication and division (without brackets) should be in the order from left to right. For example:15-2-9 = ()15-2 =1313-9 = 4 ().
In mathematics
Formula refers to the formula listed when calculating numbers (or algebraic expressions), including numbers (or letters instead of numbers) and operation symbols. According to the different calculation methods, the formulas are generally divided into horizontal and vertical. Unlike expressions, expressions are expressions that connect data of the same type (such as constants, variables, functions, etc.). ) Use operation symbols according to certain rules, and expressions are expressions that connect numbers through operation symbols to calculate results, and need an equal sign.