一颗人造地球卫星的速度等于第一宇宙速度,每昼夜绕地球转n周,地球半径为R.现欲发射一颗地球同步卫星,它应定点于赤道上空的高度是(    )

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.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。

    My friend Lin Tao was born in Liaoning Province to the northeast of China. He met with a traffic accident when he was younger. As a result, he lost one of his legs. Although he suffered a great deal, he never loses his heart. Lin Tao has been published a book. He enjoys playing his guitar and listen to music. He does excellent job in school. But he is always ready to help those in trouble. He wins the respects of his classmates and his teachers.

    In my opinion, Lin Tao is extreme brave and he sets a good example to we all.

随着国际油价的不断攀升，能源问题再次成为人们关注的焦点．发展离不开能源，下列说法正确的是（　　）

因式分解：a²﹣9=____．