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Question
If the coordinates of the points A, B, C, D be (1, 2, 3), (4, 5, 7), (−4, 3, −6) and (2, 9, 2) respectively, then find the angle between the lines AB and CD.
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Solution
The direction ratios of AB and CD are proportional to 3, 3, 4 and 6, 6, 8, respectively.
Let θ be the angle between AB and CD. Then,
\[\cos \theta = \frac{3 \times 6 + 3 \times 6 + 4 \times 8}{\sqrt{3^2 + 3^2 + 4^2} \sqrt{6^2 + 6^2 + 8^2}}\]
\[ = \frac{68}{\sqrt{34} \sqrt{136}}\]
\[ = 1\]
\[ \Rightarrow \theta = 0°\]
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