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# Find the Angle Between the Lines Whose Direction Ratios Are Proportional to A, B, C and B − C, C − A, a − B. - CBSE (Arts) Class 12 - Mathematics

ConceptDirection Cosines and Direction Ratios of a Line

#### Question

Find the angle between the lines whose direction ratios are proportional to abc and b − cc − aa− b.

#### Solution

$\text{ Let } \theta \text { be the angle between the given lines } .$

$\text{ We have }$

$a_1 = a, b_1 = b, c_1 = c$

$a_2 = b - c, b_2 = c - a, c_2 = a - b$

$\text{ Now },$

$\cos \theta = \frac{a_1 a_2 + b_1 b_2 + c_1 c_2}{\sqrt{{a_1}^2 + {b_1}^2 + {c_1}^2}\sqrt{{a_2}^2 + {b_2}^2 + {c_2}^2}}$

$= \frac{a\left( b - c \right) + b\left( c - a \right) + c\left( a - b \right)}{\sqrt{a^2 + b^2 + c^2}\sqrt{\left( b - c \right)^2 + \left( c - a \right)\left( a - b \right)}} = \frac{ab - ac + bc - ab + ac - bc}{\sqrt{a^2 + b^2 + c^2}\sqrt{\left( b - c \right)^2 + \left( c - a \right)\left( a - b \right)}} = 0$

$\Rightarrow \theta = \frac{\pi}{2}$

$\text { Thus, the angle between the given lines measures } 90° .$

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#### Video TutorialsVIEW ALL [3]

Solution Find the Angle Between the Lines Whose Direction Ratios Are Proportional to A, B, C and B − C, C − A, a − B. Concept: Direction Cosines and Direction Ratios of a Line.
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