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VECTOR ALGEBRA
459
15 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