选择正确的一项 ( )
-This is my mother.
- a teacher.
A:He's
B:She's
已知a→=(2,1),b→=(﹣3,﹣4),
(1)求2a→+3b→,|a→﹣2b→|;
(2)求a→与b→的夹角的余弦值.