C向AB做垂线，垂足D斜边长 AC = √10直角边 CD = √2sinA = √2/√10 = 1/√5 = √5/5选 B
AC/sinB=BC/sinA上面式子中AC、sinB和BC皆可求，所以sinA可求
先用余弦定理求cos再求sin噻
1楼正解
a
B
