Class CBSE Class 12 Mathematics Three Dimensional Geometry Q #1823
COMPETENCY BASED
APPLY
5 Marks 2026 AISSCE(Board Exam) LA
A line passing through the points $A(1,2,3)$ and $B(5,8,11)$ intersects the line $\vec{r}=4\hat{i}+\hat{j}+\lambda(5\hat{i}+2\hat{j}+\hat{k})$. Find the co-ordinates of the point of intersection. Hence, write the equation of a line passing through the point of intersection and perpendicular to both the lines.

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Detailed Solution

Step 1: Find the equation of line AB

The line passes through $A(1,2,3)$ and $B(5,8,11)$. The direction vector is $\vec{v_1} = (5-1)\hat{i} + (8-2)\hat{j} + (11-3)\hat{k} = 4\hat{i} + 6\hat{j} + 8\hat{k}$. Simplifying, we use the direction ratio vector $\vec{d_1} = 2\hat{i} + 3\hat{j} + 4\hat{k}$. The equation is $\vec{r} = (\hat{i} + 2\hat{j} + 3\hat{k}) + \mu(2\hat{i} + 3\hat{j} + 4\hat{k})$.

Step 2: Find the point of intersection

Equating the two lines: $(1+2\mu, 2+3\mu, 3+4\mu) = (4+5\lambda, 1+2\lambda, \lambda)$. Solving the system: $3+4\mu = \lambda$. Substituting into the first two: $1+2\mu = 4+5(3+4\mu) \Rightarrow 1+2\mu = 19+20\mu \Rightarrow -18 = 18\mu \Rightarrow \mu = -1$. Thus, the point is $(1-2, 2-3, 3-4) = (-1, -1, -1)$.

Step 3: Find the direction of the perpendicular line

The direction of the required line is the cross product of the direction vectors of the two lines: $\vec{d_1} = 2\hat{i} + 3\hat{j} + 4\hat{k}$ and $\vec{d_2} = 5\hat{i} + 2\hat{j} + \hat{k}$. $$\vec{d_3} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ 2 & 3 & 4 \\ 5 & 2 & 1 \end{vmatrix} = \hat{i}(3-8) - \hat{j}(2-20) + \hat{k}(4-15) = -5\hat{i} + 18\hat{j} - 11\hat{k}$$

Step 4: Write the final equation

The line passes through $(-1, -1, -1)$ with direction vector $-5\hat{i} + 18\hat{j} - 11\hat{k}$. The equation is $\vec{r} = (-\hat{i} - \hat{j} - \hat{k}) + t(-5\hat{i} + 18\hat{j} - 11\hat{k})$.

Final Answer: Point: (-1, -1, -1); Equation: \vec{r} = (-\hat{i} - \hat{j} - \hat{k}) + t(-5\hat{i} + 18\hat{j} - 11\hat{k})

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires the student to synthesize knowledge of vector equations, intersection of lines, and cross-product properties to solve a multi-step geometric problem.
Knowledge Dimension: PROCEDURAL
Justification: The student must follow a specific sequence of mathematical operations (finding line equations, solving systems of linear equations, and calculating cross products).
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. This question tests the student's ability to apply 3D geometry concepts beyond rote memorization, aligning with NEP 2020's focus on higher-order thinking.