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：在错的词下画一横线，并在该词下面写出修改后的词。

注意：1）处错误及其修改仅限一词；

2）只允许修改10处，多者（从第11处起）不计分。

    A good friend of me had a dog. He loved it very much that he regarded it like his best friend. Unfortunately, last Friday the dog was killed by car, which made him be heartbroken. I must did something to comfort him. To cheer him up, I went through all my old photo and made a nice picture collection of him and his dog. Hopeful, it will work. I knew I can't replace his pet and best friend but at least I can put a smile on his face again. That is how I should do for a friend. A friend in need is a friend indeed.

用反证法证明“一个三角形中不可能有两个角是钝角”

已知：△ABC

求证：∠A、∠B、∠C中不能有两个角是钝角

证明：假设．