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3D Geometry question

2008 · Shift 2 · Q26
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  5. /2008 · Shift 2 · Q26

3D Geometry question

2008 · Shift 2 · Q26

JEE AdvancedMathematics3D GeometryMCQ+3 / −1
Consider the lines, L1:x+13=y+21=z+12L2:x−21=y−22=z−33{L_1}:{{x + 1} \over 3} = {{y + 2} \over 1} = {{z + 1} \over 2}{L_2}:{{x - 2} \over 1} = {{y - 2} \over 2} = {{z - 3} \over 3}L1​:3x+1​=1y+2​=2z+1​L2​:1x−2​=2y−2​=3z−3​The distance of the point (1,1,1)(1, 1, 1)(1,1,1) from the plane passing through the point (−1,−2,−1)(-1, -2, -1)(−1,−2,−1) and whose normal is perpendicular to both the lines L1{L_1}L1​ and L2{L_2}L2​ is :
  1. A
    275{2 \over {\sqrt {75} }}75​2​
  2. B
    775{7 \over {\sqrt {75} }}75​7​
  3. C
    1375{13 \over {\sqrt {75} }}75​13​
  4. D
    2375{23 \over {\sqrt {75} }}75​23​
View written solutionFree

Correct answer: C

The user wants me to find the distance of a point from a plane. The plane is defined by a point it passes through and its normal vector, which is perpendicular to two given lines.

Step-by-step Solution:

  1. Identify the direction vectors of the given lines. The equation of the first line is L1:x+13=y+21=z+12{L_1}:{{x + 1} \over 3} = {{y + 2} \over 1} = {{z + 1} \over 2}L1​:3x+1​=1y+2​=2z+1​. The direction vector of L1{L_1}L1​ is d⃗1=3i^+1j^+2k^{\vec d_1} = 3\hat i + 1\hat j + 2\hat kd1​=3i^+1j^​+2k^.

    The equation of the second line is L2:x−21=y−22=z−33{L_2}:{{x - 2} \over 1} = {{y - 2} \over 2} = {{z - 3} \over 3}L2​:1x−2​=2y−2​=3z−3​. The direction vector of L2{L_2}L2​ is d⃗2=1i^+2j^+3k^{\vec d_2} = 1\hat i + 2\hat j + 3\hat kd2​=1i^+2j^​+3k^.

  2. Find the normal vector to the plane. The problem states that the normal to the plane, let's call it n⃗{\vec n}n, is perpendicular to both lines L1{L_1}L1​ and L2{L_2}L2​. This means n⃗{\vec n}n is perpendicular to both d⃗1{\vec d_1}d1​ and d⃗2{\vec d_2}d2​. We can find n⃗{\vec n}n by taking the cross product of d⃗1{\vec d_1}d1​ and d⃗2{\vec d_2}d2​. n⃗=d⃗1×d⃗2=∣i^j^k^312123∣{\vec n} = {\vec d_1} \times {\vec d_2} = \begin{vmatrix} \hat i & \hat j & \hat k \\ 3 & 1 & 2 \\ 1 & 2 & 3 \end{vmatrix}n=d1​×d2​=​i^31​j^​12​k^23​​ Expanding the determinant: n⃗=i^(1⋅3−2⋅2)−j^(3⋅3−1⋅2)+k^(3⋅2−1⋅1){\vec n} = \hat i(1 \cdot 3 - 2 \cdot 2) - \hat j(3 \cdot 3 - 1 \cdot 2) + \hat k(3 \cdot 2 - 1 \cdot 1)n=i^(1⋅3−2⋅2)−j^​(3⋅3−1⋅2)+k^(3⋅2−1⋅1) n⃗=i^(3−4)−j^(9−2)+k^(6−1){\vec n} = \hat i(3 - 4) - \hat j(9 - 2) + \hat k(6 - 1)n=i^(3−4)−j^​(9−2)+k^(6−1) n⃗=−1i^−7j^+5k^{\vec n} = -1\hat i - 7\hat j + 5\hat kn=−1i^−7j^​+5k^ The direction ratios of the normal are ⟨−1,−7,5⟩\langle -1, -7, 5 \rangle⟨−1,−7,5⟩.

  3. Find the equation of the plane. The plane passes through the point P(−1,−2,−1)P(-1, -2, -1)P(−1,−2,−1). The equation of a plane passing through a point (x0,y0,z0)(x_0, y_0, z_0)(x0​,y0​,z0​) with a normal vector with direction ratios ⟨A,B,C⟩\langle A, B, C \rangle⟨A,B,C⟩ is given by A(x−x0)+B(y−y0)+C(z−z0)=0A(x - x_0) + B(y - y_0) + C(z - z_0) = 0A(x−x0​)+B(y−y0​)+C(z−z0​)=0.

    Using the point P(−1,−2,−1)P(-1, -2, -1)P(−1,−2,−1) and the direction ratios of the normal ⟨−1,−7,5⟩\langle -1, -7, 5 \rangle⟨−1,−7,5⟩: −1(x−(−1))−7(y−(−2))+5(z−(−1))=0-1(x - (-1)) - 7(y - (-2)) + 5(z - (-1)) = 0−1(x−(−1))−7(y−(−2))+5(z−(−1))=0 −1(x+1)−7(y+2)+5(z+1)=0-1(x + 1) - 7(y + 2) + 5(z + 1) = 0−1(x+1)−7(y+2)+5(z+1)=0 −x−1−7y−14+5z+5=0-x - 1 - 7y - 14 + 5z + 5 = 0−x−1−7y−14+5z+5=0 −x−7y+5z−10=0-x - 7y + 5z - 10 = 0−x−7y+5z−10=0 Multiplying the entire equation by −1-1−1 to make the coefficient of xxx positive: x+7y−5z+10=0x + 7y - 5z + 10 = 0x+7y−5z+10=0

  4. Calculate the distance of the point (1, 1, 1) from the plane. The distance of a point (x1,y1,z1)(x_1, y_1, z_1)(x1​,y1​,z1​) from a plane Ax+By+Cz+D=0Ax + By + Cz + D = 0Ax+By+Cz+D=0 is given by the formula: d=∣Ax1+By1+Cz1+D∣A2+B2+C2d = {{|Ax_1 + By_1 + Cz_1 + D|} \over {\sqrt{A^2 + B^2 + C^2}}}d=A2+B2+C2​∣Ax1​+By1​+Cz1​+D∣​ The point is Q(1,1,1)Q(1, 1, 1)Q(1,1,1) and the plane is x+7y−5z+10=0x + 7y - 5z + 10 = 0x+7y−5z+10=0. Here, A=1A=1A=1, B=7B=7B=7, C=−5C=-5C=−5, D=10D=10D=10, and (x1,y1,z1)=(1,1,1)(x_1, y_1, z_1) = (1, 1, 1)(x1​,y1​,z1​)=(1,1,1).

    d=∣1(1)+7(1)−5(1)+10∣12+72+(−5)2d = {{|1(1) + 7(1) - 5(1) + 10|} \over {\sqrt{1^2 + 7^2 + (-5)^2}}}d=12+72+(−5)2​∣1(1)+7(1)−5(1)+10∣​ d=∣1+7−5+10∣1+49+25d = {{|1 + 7 - 5 + 10|} \over {\sqrt{1 + 49 + 25}}}d=1+49+25​∣1+7−5+10∣​ d=∣13∣75d = {{|13|} \over {\sqrt{75}}}d=75​∣13∣​ d=1375d = {{13} \over {\sqrt{75}}}d=75​13​

  5. Conclusion. The calculated distance is 1375{13 \over {\sqrt{75}}}75​13​. Comparing this with the given options, it matches option C.

    A: 275{2 \over {\sqrt {75} }}75​2​ B: 775{7 \over {\sqrt {75} }}75​7​ C: 1375{13 \over {\sqrt {75} }}75​13​ D: 2375{23 \over {\sqrt {75} }}75​23​

The correct option is C.

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