Question

Using vector method, prove that the following points are collinear:
A (2, −1, 3), B (4, 3, 1) and C (3, 1, 2)

Answer 1

Given the points \[A\left( 2, - 1, 3 \right), B\left( 4, 3, 1 \right)\] and \[C\left( 3, 1, 2 \right)\]. Then,
\[\overrightarrow{AB} =\]  Position vector of B - Position vector of A
\[= 4 \hat{i} + 3 \hat{j} + \hat{k} - 2 \hat{i} + \hat{j} - 3 \hat{k} \]
\[ = 2 \hat{i} + 4 \hat{j} - 2 \hat{k} \]
\[ = - 2\left( - \hat{i} - 2 \hat{j} + \hat{k} \right)\]
\[\overrightarrow{BC} =\] Position vector of C - Position vector of B
\[= 3 \hat{i} + \hat{j} + 2 \hat{k} - 4 \hat{i} - 3 \hat{j} - \hat{k} \]
\[ = - \hat{i} - 2 \hat{j} + \hat{k}\]
\[\therefore \overrightarrow{AB} = - 2 \overrightarrow{BC}\]
\[So, \overrightarrow{AB} , \overrightarrow{BC}\]  are parallel vectors. But  B is a point common to them.
Hence, The given points A, B and C  are collinear.