—He _______ to the library. I _______ him there five minutes ago.
A:has been; have met
B:has been; met
C:has gone; have met
D:has gone; met
当a=3,b=﹣1时
(1)求代数式a2﹣b2和(a+b)(a﹣b)的值;
(2)猜想这两个代数式的值有何关系?
(3)根据(1)(2),你能用简便方法算出a=2008,b=2007时,a2﹣b2的值吗?
关于图中天体系统的名称,标注正确的选项为( )