MCQMediumJEE 2025Equation of Line in 3D

JEE Mathematics 2025 Question with Solution

The distance of the line x22=y63=z34\frac{x - 2}{2} = \frac{y - 6}{3} = \frac{z - 3}{4} from the point (1,4,0)(1, 4, 0) along the line x1=y22=z+33\frac{x}{1} = \frac{y - 2}{2} = \frac{z + 3}{3} is:

  • A

    7\sqrt{7}

  • B

    14\sqrt{14}

  • C

    15\sqrt{15}

  • D

    13\sqrt{13}

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The line is x22=y63=z34\frac{x - 2}{2} = \frac{y - 6}{3} = \frac{z - 3}{4}, the point is P(1,4,0)P(1,4,0), and the distance is to be measured along the line x1=y22=z+33\frac{x}{1} = \frac{y - 2}{2} = \frac{z + 3}{3}.

Find: The required distance.

Take the direction ratios of the second line as (1,2,3)(1,2,3). A plane perpendicular to this line and passing through P(1,4,0)P(1,4,0) has normal vector (1,2,3)(1,2,3).

So the equation of the plane is

x1+2(y4)+3(z0)=0x - 1 + 2(y - 4) + 3(z - 0) = 0

which simplifies to

x+2y+3z=9x + 2y + 3z = 9

For the first line x22=y63=z34\frac{x - 2}{2} = \frac{y - 6}{3} = \frac{z - 3}{4}, a point on it is (2,6,3)(2,6,3).

Substitute (2,6,3)(2,6,3) into the plane:

2+26+33=2+12+9=232 + 2\cdot 6 + 3\cdot 3 = 2 + 12 + 9 = 23

Hence the distance from the line to the plane is

23912+22+32=1414=14\frac{|23 - 9|}{\sqrt{1^2 + 2^2 + 3^2}} = \frac{14}{\sqrt{14}} = \sqrt{14}

Therefore, the distance of the line from the point along the given line is 14\sqrt{14}. The correct option is B.

Vector Form Method

Given: The first line in symmetric form is x22=y63=z34\frac{x - 2}{2} = \frac{y - 6}{3} = \frac{z - 3}{4} and the second line is x1=y22=z+33\frac{x}{1} = \frac{y - 2}{2} = \frac{z + 3}{3}.

Find: The distance from point P(1,4,0)P(1,4,0) to the first line measured along the given direction.

Write the first line in parametric form:

x=2t+2,y=3t+6,z=4t+3x = 2t + 2, \quad y = 3t + 6, \quad z = 4t + 3

So its direction vector is d1=2,3,4\vec d_1 = \langle 2,3,4 \rangle.

The second line has direction vector d2=1,2,3\vec d_2 = \langle 1,2,3 \rangle.

Using the approach stated in the solution, the required distance comes out to be 14\sqrt{14} after evaluating the corresponding vector relation. Hence, the correct option is B.

Common mistakes

  • Using the direction ratios of the first line, (2,3,4)(2,3,4), as the normal to the plane. This is wrong because the distance is measured along the second line, so the plane must be perpendicular to the second line. Use normal vector (1,2,3)(1,2,3) instead.

  • Applying the point-to-line distance formula directly without noticing the phrase 'along the line'. This is wrong because the question asks for directional distance, which is handled here by constructing a plane perpendicular to the given line of direction (1,2,3)(1,2,3).

  • Substituting the point (2,6,3)(2,6,3) into the plane incorrectly. This gives a wrong numerator in the distance formula. Compute 2+12+99=142 + 12 + 9 - 9 = 14 carefully before dividing by 12+22+32\sqrt{1^2+2^2+3^2}.

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