alleryTwo vectors are PERPENDICULAR iff:
A|ā| = |b̄|
Bā × b̄ = 0
Cā · b̄ = 0
Dā + b̄ = 0
Answer & Solution
Correct answer: C. ā · b̄ = 0
Dot product ā · b̄ = |ā||b̄|cos θ = 0 iff cos θ = 0 iff θ = 90°.
So zero dot product = perpendicular vectors.
(Zero cross product = parallel/collinear vectors.)
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