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。

    As we all know, we can meet with various challenges in our life. Faced with challenges, how should we do?

    Last month, our class climbed Mount Tai, that is more than 1,500 meters above sea level. Lacking exercise, I thought that this activity was great challenge for me. I felt very exhausted halfway that I wanted to give up. See this, my classmates and even some strangers encourage me. By the end, I managed to reach on the top of the mountain.

    Therefore, don't give up in face of challenges. If you are determined enough, it's certainly that you can overcome it.

等腰三角形的底和腰是方程x²-6x+8=0的两个根，则这个三角形的周长是（   ）