17.已知空间三点 A(-4,0,4) .(-2,2,4) .C(-3,2,3) .设 overrightarrow (a)=overrightarrow (AB) .b=BC-|||-(1)求|a|,→(b);-|||-(2)求a与b的夹角;-|||-(3)若向量 overrightarrow (a)+overrightarrow (b) 与 overrightarrow (a)-2overrightarrow (b) 互相垂直,求实数k的值.