四、四则运算的性质 小学数学课本第六册第62页对于四则混合运算的顺序是这样规定的:“在一个没有括号的算式里,如果只有加、减法,或者只有乘、除法,要从左往右依次演算;如果既有加、减法,又有乘、除法,要先算乘除法,后算加减法。”这样规定是有道理的。因为由实际问题列出的四则算式,通常要求先乘除后加减。例如第四册课本第70页例1,食堂原有大米50 The Nature of Four, Four Operations Elementary Math Text Book, Volume 6, p. 62 The order of the four hybrid operations is specified as follows: “In an unbracketed formula, if there is only addition, subtraction, or multiply and divide, If you have both additions and subtractions, multiplication and division, you must first calculate and divide, and then add and subtract. ”Such a rule makes sense. Because of the four equations listed by practical problems, it is usually required to multiply and divide by addition and subtraction first. For example, Volume 4, Textbook, p. 70, Example 1, Original 50-meter canteen