Class CBSE Class 12 Mathematics Three Dimensional Geometry Q #1375
COMPETENCY BASED
UNDERSTAND
5 Marks 2025 AISSCE(Board Exam) LA
Find a point P on the line $\frac{x+5}{1}=\frac{y+3}{4}=\frac{z-6}{-9}$ such that its distance from point $Q(2,4,-1)$ is 7 units. Also, find the equation of line joining P and Q.

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Detailed Solution<\/h3>\r\n <\/div>\r\n\r\n
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Step 1: Parametric Representation of Line

Let the given line be represented in parametric form. We introduce a parameter $\lambda$ such that:

$\frac{x+5}{1}=\frac{y+3}{4}=\frac{z-6}{-9} = \lambda$

From this, we can express the coordinates of a general point P on the line as:

$x = \lambda - 5$

$y = 4\lambda - 3$

$z = -9\lambda + 6$

So, the coordinates of point P are $(\lambda - 5, 4\lambda - 3, -9\lambda + 6)$.

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Step 2: Distance Calculation

We are given that the distance between point P and point Q(2, 4, -1) is 7 units. We use the distance formula:

$PQ = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2}$

Substituting the coordinates of P and Q, we get:

$7 = \sqrt{(\lambda - 5 - 2)^2 + (4\lambda - 3 - 4)^2 + (-9\lambda + 6 - (-1))^2}$

$7 = \sqrt{(\lambda - 7)^2 + (4\lambda - 7)^2 + (-9\lambda + 7)^2}$

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Step 3: Squaring and Simplifying

Squaring both sides of the equation, we get:

$49 = (\lambda - 7)^2 + (4\lambda - 7)^2 + (-9\lambda + 7)^2$

$49 = (\lambda^2 - 14\lambda + 49) + (16\lambda^2 - 56\lambda + 49) + (81\lambda^2 - 126\lambda + 49)$

$49 = 98\lambda^2 - 196\lambda + 147$

$0 = 98\lambda^2 - 196\lambda + 98$

Dividing by 98, we get:

$0 = \lambda^2 - 2\lambda + 1$

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Step 4: Solving for λ

The quadratic equation is:

$\lambda^2 - 2\lambda + 1 = 0$

$(\lambda - 1)^2 = 0$

Therefore, $\lambda = 1$.

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Step 5: Finding Coordinates of P

Substitute $\lambda = 1$ into the coordinates of P:

$x = 1 - 5 = -4$

$y = 4(1) - 3 = 1$

$z = -9(1) + 6 = -3$

So, the coordinates of point P are $(-4, 1, -3)$.

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Step 6: Finding the Equation of Line PQ

The equation of the line joining two points $P(x_1, y_1, z_1)$ and $Q(x_2, y_2, z_2)$ is given by:

$\frac{x - x_1}{x_2 - x_1} = \frac{y - y_1}{y_2 - y_1} = \frac{z - z_1}{z_2 - z_1}$

Here, $P(-4, 1, -3)$ and $Q(2, 4, -1)$. Substituting these values, we get:

$\frac{x - (-4)}{2 - (-4)} = \frac{y - 1}{4 - 1} = \frac{z - (-3)}{-1 - (-3)}$

$\frac{x + 4}{6} = \frac{y - 1}{3} = \frac{z + 3}{2}$

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\r\n Final Answer: P(-4, 1, -3) and Equation of line PQ: $\frac{x + 4}{6} = \frac{y - 1}{3} = \frac{z + 3}{2}$<\/span>\r\n <\/p>\r\n <\/div>\r\n <\/div>

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\r\n Pedagogical Audit<\/span>\r\n <\/div>\r\n
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\r\n Bloom's Analysis:<\/span> \r\n This is an APPLY<\/span> question because the student needs to apply the concepts of 3D geometry, distance formula, and equation of a line in space to solve the problem.\r\n <\/div>\r\n
\r\n Knowledge Dimension:<\/span> \r\n PROCEDURAL<\/span>\r\n
\r\n Justification:<\/span> \r\n The question requires the student to follow a specific procedure to find the point P and then use another procedure to find the equation of the line PQ.<\/span>\r\n <\/div>\r\n <\/div>\r\n
\r\n Syllabus Audit:<\/span> \r\n In the context of CBSE Class 12, this is classified as APPLICATION<\/span>. The question requires the application of concepts related to 3D geometry, including lines in space and the distance formula. It goes beyond simple recall and understanding, requiring students to apply these concepts in a problem-solving context.\r\n <\/div>\r\n <\/div>\r\n <\/div>