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3D Geometry question

2008 · Shift 1 · Q26
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  5. /2008 · Shift 1 · Q26

3D Geometry question

2008 · Shift 1 · Q26

JEE AdvancedMathematics3D GeometryMCQ+4 / −1
Consider three planes P1:x−y+z=1P2:x+y−z=1P3:x−3y+3z=2{P_1}:x - y + z = 1{P_2}:x + y - z = 1{P_3}:x - 3y + 3z = 2P1​:x−y+z=1P2​:x+y−z=1P3​:x−3y+3z=2 Let L1,L2,L3{L_1},{L_2},{L_3}L1​,L2​,L3​ be the lines of intersection of the planes P2{P_2}P2​ and P3,P3{P_3},{P_3}P3​,P3​ and P1,P1{P_1},{P_1}P1​,P1​ and P2,{P_2},P2​, respectively. STATEMENT - 1Z: At least two of the lines L1,L2{L_1},{L_2}L1​,L2​ and L3{L_3}L3​ are non-parallel and STATEMENT - 2: The three planes doe not have a common point.
  1. A
    STATEMENT - 1 is True, STATEMENT - 2 is True; STATEMENT - 2 is a correct explanation for STATEMENT - 1
  2. B
    STATEMENT - 1 is True, STATEMENT - 2 is True; STATEMENT - 2 is NOT a correct explanation for STATEMENT - 1
  3. C
    STATEMENT - 1 is True, STATEMENT - 2 is False
  4. D
    STATEMENT - 1 is False, STATEMENT - 2 is True
View written solutionFree

Correct answer: D

The user wants me to analyze two statements regarding three planes and their lines of intersection.

The given planes are: P1:x−y+z=1{P_1}: x - y + z = 1P1​:x−y+z=1 P2:x+y−z=1{P_2}: x + y - z = 1P2​:x+y−z=1 P3:x−3y+3z=2{P_3}: x - 3y + 3z = 2P3​:x−3y+3z=2

Let the normal vectors to the planes be n1⃗,n2⃗,n3⃗\vec{n_1}, \vec{n_2}, \vec{n_3}n1​​,n2​​,n3​​ respectively. n1⃗=⟨1,−1,1⟩\vec{n_1} = \langle 1, -1, 1 \ranglen1​​=⟨1,−1,1⟩ n2⃗=⟨1,1,−1⟩\vec{n_2} = \langle 1, 1, -1 \ranglen2​​=⟨1,1,−1⟩ n3⃗=⟨1,−3,3⟩\vec{n_3} = \langle 1, -3, 3 \ranglen3​​=⟨1,−3,3⟩

Let L1,L2,L3{L_1}, {L_2}, {L_3}L1​,L2​,L3​ be the lines of intersection of the planes P2{P_2}P2​ and P3{P_3}P3​, P3{P_3}P3​ and P1{P_1}P1​, and P1{P_1}P1​ and P2{P_2}P2​ respectively.

Analysis of Statement 2: The three planes do not have a common point.

To check for a common point, we need to solve the system of linear equations:

  1. x−y+z=1x - y + z = 1x−y+z=1
  2. x+y−z=1x + y - z = 1x+y−z=1
  3. x−3y+3z=2x - 3y + 3z = 2x−3y+3z=2

Step 1: Solve the system of equations. We can add equation (1) and equation (2): (x−y+z)+(x+y−z)=1+1(x - y + z) + (x + y - z) = 1 + 1(x−y+z)+(x+y−z)=1+1 2x=2  ⟹  x=12x = 2 \implies x = 12x=2⟹x=1

Substitute x=1x = 1x=1 into equations (1) and (2): From (1): 1−y+z=1  ⟹  z=y1 - y + z = 1 \implies z = y1−y+z=1⟹z=y From (2): 1+y−z=1  ⟹  y=z1 + y - z = 1 \implies y = z1+y−z=1⟹y=z Both equations give the same relationship, y=zy = zy=z.

Now substitute x=1x=1x=1 and z=yz=yz=y into equation (3): 1−3y+3(y)=21 - 3y + 3(y) = 21−3y+3(y)=2 1−3y+3y=21 - 3y + 3y = 21−3y+3y=2 1=21 = 21=2

Step 2: Interpret the result. The result 1=21 = 21=2 is a contradiction. This means the system of equations is inconsistent and has no solution. Therefore, the three planes do not have a common point of intersection.

Conclusion for Statement 2: STATEMENT - 2 is True.

Analysis of Statement 1: At least two of the lines L1,L2{L_1},{L_2}L1​,L2​ and L3{L_3}L3​ are non-parallel.

The direction vector of the line of intersection of two planes is given by the cross product of their normal vectors.

Step 1: Find the direction vector of line L1. L1{L_1}L1​ is the intersection of P2{P_2}P2​ and P3{P_3}P3​. Its direction vector d1⃗\vec{d_1}d1​​ is parallel to n2⃗×n3⃗\vec{n_2} \times \vec{n_3}n2​​×n3​​. d1⃗=n2⃗×n3⃗=∣ijk11−11−33∣=i(3−3)−j(3−(−1))+k(−3−1)=0i−4j−4k=⟨0,−4,−4⟩\vec{d_1} = \vec{n_2} \times \vec{n_3} = \begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ 1 & 1 & -1 \\ 1 & -3 & 3 \end{vmatrix} = \mathbf{i}(3-3) - \mathbf{j}(3 - (-1)) + \mathbf{k}(-3 - 1) = 0\mathbf{i} - 4\mathbf{j} - 4\mathbf{k} = \langle 0, -4, -4 \rangled1​​=n2​​×n3​​=​i11​j1−3​k−13​​=i(3−3)−j(3−(−1))+k(−3−1)=0i−4j−4k=⟨0,−4,−4⟩ So, d1⃗\vec{d_1}d1​​ is parallel to ⟨0,1,1⟩\langle 0, 1, 1 \rangle⟨0,1,1⟩.

Step 2: Find the direction vector of line L2. L2{L_2}L2​ is the intersection of P3{P_3}P3​ and P1{P_1}P1​. Its direction vector d2⃗\vec{d_2}d2​​ is parallel to n3⃗×n1⃗\vec{n_3} \times \vec{n_1}n3​​×n1​​. d2⃗=n3⃗×n1⃗=∣ijk1−331−11∣=i(−3−(−3))−j(1−3)+k(−1−(−3))=0i+2j+2k=⟨0,2,2⟩\vec{d_2} = \vec{n_3} \times \vec{n_1} = \begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ 1 & -3 & 3 \\ 1 & -1 & 1 \end{vmatrix} = \mathbf{i}(-3 - (-3)) - \mathbf{j}(1 - 3) + \mathbf{k}(-1 - (-3)) = 0\mathbf{i} + 2\mathbf{j} + 2\mathbf{k} = \langle 0, 2, 2 \rangled2​​=n3​​×n1​​=​i11​j−3−1​k31​​=i(−3−(−3))−j(1−3)+k(−1−(−3))=0i+2j+2k=⟨0,2,2⟩ So, d2⃗\vec{d_2}d2​​ is parallel to ⟨0,1,1⟩\langle 0, 1, 1 \rangle⟨0,1,1⟩.

Step 3: Find the direction vector of line L3. L3{L_3}L3​ is the intersection of P1{P_1}P1​ and P2{P_2}P2​. Its direction vector d3⃗\vec{d_3}d3​​ is parallel to n1⃗×n2⃗\vec{n_1} \times \vec{n_2}n1​​×n2​​. d3⃗=n1⃗×n2⃗=∣ijk1−1111−1∣=i(1−1)−j(−1−1)+k(1−(−1))=0i+2j+2k=⟨0,2,2⟩\vec{d_3} = \vec{n_1} \times \vec{n_2} = \begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ 1 & -1 & 1 \\ 1 & 1 & -1 \end{vmatrix} = \mathbf{i}(1-1) - \mathbf{j}(-1 - 1) + \mathbf{k}(1 - (-1)) = 0\mathbf{i} + 2\mathbf{j} + 2\mathbf{k} = \langle 0, 2, 2 \rangled3​​=n1​​×n2​​=​i11​j−11​k1−1​​=i(1−1)−j(−1−1)+k(1−(−1))=0i+2j+2k=⟨0,2,2⟩ So, d3⃗\vec{d_3}d3​​ is parallel to ⟨0,1,1⟩\langle 0, 1, 1 \rangle⟨0,1,1⟩.

Step 4: Compare the direction vectors. We have found that d1⃗∥⟨0,1,1⟩\vec{d_1} \parallel \langle 0, 1, 1 \rangled1​​∥⟨0,1,1⟩, d2⃗∥⟨0,1,1⟩\vec{d_2} \parallel \langle 0, 1, 1 \rangled2​​∥⟨0,1,1⟩, and d3⃗∥⟨0,1,1⟩\vec{d_3} \parallel \langle 0, 1, 1 \rangled3​​∥⟨0,1,1⟩. This means that all three lines of intersection, L1,L2,L3{L_1}, {L_2}, {L_3}L1​,L2​,L3​, are parallel to each other.

Conclusion for Statement 1: The statement says "At least two of the lines L1,L2{L_1},{L_2}L1​,L2​ and L3{L_3}L3​ are non-parallel". Since all three lines are parallel, this statement is False.

Final Evaluation

  • STATEMENT - 1 is False.
  • STATEMENT - 2 is True.

This corresponds to option D.

Geometrically, since the three planes do not have a common point and their lines of intersection are parallel, they form a triangular prism.

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