Class CBSE Class 12 Mathematics Three Dimensional Geometry Q #1456
KNOWLEDGE BASED
REMEMBER
3 Marks 2025 AISSCE(Board Exam) SA
Find the distance of the point $P(2,4,-1)$ from the line $\frac{x+5}{1}=\frac{y+3}{4}=\frac{z-6}{-9}$.

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Detailed Solution

Step 1: Find a general point on the line

Let the given line be $L$. A general point on the line $L$ can be represented as: $$ \frac{x+5}{1}=\frac{y+3}{4}=\frac{z-6}{-9} = \lambda $$ So, $x = \lambda - 5$, $y = 4\lambda - 3$, and $z = -9\lambda + 6$. Thus, any point $Q$ on the line $L$ can be written as $Q(\lambda - 5, 4\lambda - 3, -9\lambda + 6)$.

Step 2: Find the direction ratios of PQ

Let $P(2,4,-1)$ be the given point. The direction ratios of the line segment $PQ$ are given by: $$ (\lambda - 5 - 2, 4\lambda - 3 - 4, -9\lambda + 6 - (-1)) = (\lambda - 7, 4\lambda - 7, -9\lambda + 7) $$

Step 3: Apply the perpendicularity condition

Since we want to find the shortest distance, $PQ$ must be perpendicular to the line $L$. The direction ratios of line $L$ are $(1, 4, -9)$. For $PQ$ to be perpendicular to $L$, the dot product of their direction ratios must be zero: $$ 1(\lambda - 7) + 4(4\lambda - 7) - 9(-9\lambda + 7) = 0 $$ $$ \lambda - 7 + 16\lambda - 28 + 81\lambda - 63 = 0 $$ $$ 98\lambda - 98 = 0 $$ $$ \lambda = 1 $$

Step 4: Find the coordinates of point Q

Substitute $\lambda = 1$ into the coordinates of point $Q$: $$ Q(1 - 5, 4(1) - 3, -9(1) + 6) = Q(-4, 1, -3) $$

Step 5: Calculate the distance PQ

Now, find the distance between $P(2,4,-1)$ and $Q(-4,1,-3)$: $$ PQ = \sqrt{(-4 - 2)^2 + (1 - 4)^2 + (-3 - (-1))^2} $$ $$ PQ = \sqrt{(-6)^2 + (-3)^2 + (-2)^2} $$ $$ PQ = \sqrt{36 + 9 + 4} $$ $$ PQ = \sqrt{49} $$ $$ PQ = 7 $$

Final Answer: 7

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Pedagogical Audit
Bloom's Analysis: This is an REMEMBER question because it requires recalling the formula for the distance between a point and a line in 3D space and applying it to the given values.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the student to follow a specific procedure to find the distance, including finding a general point on the line, determining the direction ratios, and applying the perpendicularity condition.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. It directly tests a concept taught in the textbook. The question assesses the student's ability to apply the formula for the distance between a point and a line in 3D geometry, a standard topic in the syllabus.