Unit 1: Four arithmetic knowledge points 1: In the formula without brackets, if there is only addition and subtraction, it is calculated from left to right in turn. Knowledge point 2: in the formula without brackets, if there is only multiplication and division, it is calculated from left to right in order. Summary: the same level operation is calculated from left to right (that is, addition, subtraction, multiplication and division are the same level). Knowledge point 3: In the formula containing addition, subtraction, multiplication and division (excluding brackets), multiplication and division should be calculated first, and then addition and subtraction should be calculated. Summary: multiply and divide first and then add and subtract. Knowledge point 4: If there are brackets in the formula, first count the brackets. Summary: If there are brackets, count them first. Knowledge points are inseparable at 5: 0. Summary: a number plus the original number 0, a number MINUS the original number 0, a number multiplied by 0 to get 0, 0 divided by any number of 0, 0 can not be divided (meaningless)