对输入的算术表达式计算结果，要求能识别常见函数，能自定义并存储变量，能设置实数显示方式（常规显示，科学计数法）和精度，能进行分数运算 -The input of the arithmetic expression results, hoping that they can identify common functions that can be customized and stored in a variable that can set the display of real numb