If a line passes through points with position vectors $\bar a$ and $\bar b$, then its vector equation is
A$\bar r=\bar a+\lambda(\bar b-\bar a)$
B$\bar r=\lambda(\bar a+\bar b)$
C$\bar r=\bar a\times\bar b$
D$\bar r=\bar b+\lambda\bar a$
Answer & Solution
Correct answer: A. $\bar r=\bar a+\lambda(\bar b-\bar a)$
The line through the points $\bar a$ and $\bar b$ has direction vector $\bar b-\bar a$. Hence its vector equation is $\bar r=\bar a+\lambda(\bar b-\bar a)$.
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