Two non-zero vectors $\vec a$ and $\vec b$ are perpendicular if and only if:
A$\vec a\cdot\vec b = 0$
B$|\vec a| = |\vec b|$
C$\vec a\times\vec b = 0$
D$\vec a = \vec b$
Answer & Solution
Correct answer: A. $\vec a\cdot\vec b = 0$
Non-zero vectors are perpendicular exactly when their dot product is zero.
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