Prove that  
2
2
(
) (
) |
|
| |
a
b
a
b
a
b
+
⋅
+
=
+
r
r
r
r
r
r
, if and only if ,a b
rr
 are perpendicular,
given 
0,
0
a
b
≠
rr≠
r
r Choose the correct answer in Exercises 16 to 19 16 If θ is the angle between two vectors 
a and 
rb
r
, then 
0
a b
⋅
rr≥
 only when
(A) 0
< θ <π2
(B) 0
π2
≤ θ ≤
(C) 0 < θ < π
(D) 0 ≤ θ ≤ π
17