According to the operation sequence, the calculation process is expressed by equations in turn. Three-level recursive equation calculation, that is, disjoint calculation, means that the calculation process is expressed by equations in turn according to the operation order. Calculation method: from left to right, first calculate brackets, then multiply and divide, and finally add and subtract. Addition, subtraction, multiplication and division are collectively referred to as elementary arithmetic, in which addition and subtraction are called first-level operations; Multiplication and division are called secondary operations.