When you type information manually in the Commands window, note that a number of operators require spaces for correct structure. This is especially true when your operators are supplied with values instead of placeholders, for example, lim a_{n}=a.
当您在命令窗口中手动输入时要注意，对于很多运算符来说，空格符对于保证结构的正确是必不可少的。当您在运算符中加上数值而不是占位符时，尤其要注意，例如：lim a_{n}=a。