A:how you have observed
B:how what you have observed
C:that you have observed
D:how that you have observed
(Ⅰ)试用 a→ , b→ 表示 AM→ ;
(Ⅱ)若| a→ |=5,| b→ |=3,sin∠BAC= 45 ,求中线AM的长.