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Show that the Line Through Points (4, 7, 8) and (2, 3, 4) is Parallel to the Line Through the Points (−1, −2, 1) and (1, 2, 5). - CBSE (Arts) Class 12 - Mathematics

ConceptDirection Cosines and Direction Ratios of a Line

Question

Show that the line through points (4, 7, 8) and (2, 3, 4) is parallel to the line through the points (−1, −2, 1) and (1, 2, 5).

Solution

$\text { We know that the direction ratios of the line passing through the points } \left( x_1 , y_1 , z_1 \right) \text { and } \left( x_2 , y_2 , z_2 \right) \text { are } x_2 - x_1 , y_2 - y_1 , z_2 - z_1 .$

$\text{ Let the first two points be A } \left( 4, 7, 8 \right) \text{ and } B \left( 2, 3, 4 \right) .$

$\text{ Thus, the direction ratios of AB are } \left( 2 - 4 \right), \left( 3 - 7 \right), \left( 4 - 8 \right), \text{ i . e } . - 2, - 4, - 4 .$

$\text{ Similarly, let the other two points be C } \left( - 1, - 2, 1 \right) \text{ and } D\left( 1, 2, 5 \right) .$

$\text{ Thus, the direction ratios of CD are } \left[ 1 - \left( - 1 \right) \right], \left[ 2 - \left( - 2 \right) \right], \left( 5 - 1 \right),\text{ i . e} . 2, 4, 4 .$

$\text{ It can be seen that the direction ratios of CD are - 1 times that of AB, i . e . they are proportional }.$

$\text { Therefore, AB and CD are parallel lines } .$

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Solution Show that the Line Through Points (4, 7, 8) and (2, 3, 4) is Parallel to the Line Through the Points (−1, −2, 1) and (1, 2, 5). Concept: Direction Cosines and Direction Ratios of a Line.
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