下面短文中共有10处错误，每句中最多有两处，请你对其进行修改。错误涉及一个单词的添加、删除或修改。

添加：在缺词处加一个漏词符号（ ∧ ），并在此符号下面写出该加的词。

删除：把多余的词用斜线（ \ ）划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1、每处错误及其修改均仅限一词；

2、只允许修改10处，多者（从第11处起）不计分。

One of the difficulty for foreign students in American universities is taking notes while listen to lectures. These notes are later used to studying for exams. If the notes aren’t well, it will be hard for the students to prepare for the exams. But it is not easy take good notes. If you are try to write every word the professor say, you can’t keep pace with the lecture. That you should learn is how to set down only the most important words, not whole sentences. Words such like nouns and verbs are most important. Remembering that you need information, not sentences.

给出一组数据11、8、10、9、12，则这组数组的极差是______．