增加：在缺词处加一个漏字符号（∧），并在其下面写出该加的词。

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。

    Last week, our teacher offered us an answer to a difficulty maths problem in class. Though an answer seemed a little strange, but nobody doubted it except me. Thinking that his answer might wrong, I carefully studied the problem and tried to work them out in a different way. Half an hour late, I managed to find the correct answer. When I show my answer to him, my teacher praised me of my independent thinking.

    From this experiences I have learnt that questioning can serve as a bridge that help us to find the truth.

补全表格中所缺信息。

从1、2、3、4、5、6、7、8、9、10这十个数中随机选一个数替代二次根式$\sqrt{x^2-21}$中的字母x使所得二次根式有意义的概率是 ____．