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3D Geometry question

2023 · Shift 1 · Q33
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3D Geometry question

2023 · Shift 1 · Q33

JEE AdvancedMathematics3D GeometryMCQ+3 / −1
Let ℓ1\ell_1ℓ1​ and ℓ2\ell_2ℓ2​ be the lines r⃗1=λ(i^+j^+k^)\vec{r}_1=\lambda(\hat{i}+\hat{j}+\hat{k})r1​=λ(i^+j^​+k^) and r⃗2=(j^−k^)+μ(i^+k^)\vec{r}_2=(\hat{j}-\hat{k})+\mu(\hat{i}+\hat{k})r2​=(j^​−k^)+μ(i^+k^), respectively. Let XXX be the set of all the planes HHH that contain the line ℓ1\ell_1ℓ1​. For a plane HHH, let d(H)d(H)d(H) denote the smallest possible distance between the points of ℓ2\ell_2ℓ2​ and HHH. Let H0H_0H0​ be a plane in XXX for which d(H0)d\left(H_0\right)d(H0​) is the maximum value of d(H)d(H)d(H) as HHH varies over all planes in XXX.

Match each entry in List-I to the correct entries in List-II.

List - I List - II
(P) The value of d(H0)d\left(H_0\right)d(H0​) is (1) 3\sqrt{3}3​
(Q) The distance of the point (0,1,2)(0,1,2)(0,1,2) from H0H_0H0​ is (2) 13\frac{1}{\sqrt{3}}3​1​
(R) The distance of origin from H0H_0H0​ is (3) 0
(S) The distance of origin from the point of intersection of planes y=z,x=1y=z, x=1y=z,x=1 and H0H_0H0​ is (4) 2\sqrt{2}2​
(5) 12\frac{1}{\sqrt{2}}2​1​

The correct option is:
  1. A
    (P)→(2)(Q)→(4)(R)→(5)(S)→(1)(P) \rightarrow(2) \quad(Q) \rightarrow(4) \quad(R) \rightarrow(5) \quad(S) \rightarrow(1)(P)→(2)(Q)→(4)(R)→(5)(S)→(1)
  2. B
    (P)→(5)(Q)→(4)(R)→(3)(S)→(1)(P) \rightarrow(5) \quad(Q) \rightarrow(4) \quad(R) \rightarrow(3) \quad(S) \rightarrow(1)(P)→(5)(Q)→(4)(R)→(3)(S)→(1)
  3. C
    (P)→(2)(Q)→(1)(R)→(3)(S)→(2)(P) \rightarrow(2) \quad(Q) \rightarrow(1) \quad(R) \rightarrow(3) \quad(S) \rightarrow(2)(P)→(2)(Q)→(1)(R)→(3)(S)→(2)
  4. D
    (P)→(5)(Q)→(1)(R)→(4)(S)→(2)(P) \rightarrow(5) \quad(Q) \rightarrow(1) \quad(R) \rightarrow(4) \quad(S) \rightarrow(2)(P)→(5)(Q)→(1)(R)→(4)(S)→(2)
View written solutionFree

Correct answer: B

  1. Given lines

    ℓ1:r⃗=λ(i^+j^+k^)\ell_1: \vec r=\lambda(\hat i+\hat j+\hat k)ℓ1​:r=λ(i^+j^​+k^) so ℓ1\ell_1ℓ1​ passes through origin with direction vector a⃗=(1,1,1).\vec a=(1,1,1).a=(1,1,1).

    ℓ2:(x,y,z)=(0,1,−1)+μ(1,0,1)\ell_2:(x,y,z)=(0,1,-1)+\mu(1,0,1)ℓ2​:(x,y,z)=(0,1,−1)+μ(1,0,1) since j^−k^=(0,1,−1)\hat j-\hat k=(0,1,-1)j^​−k^=(0,1,−1) and direction vector is b⃗=(1,0,1).\vec b=(1,0,1).b=(1,0,1).

  2. General plane containing ℓ1\ell_1ℓ1​

    Any plane H∈XH\in XH∈X containing ℓ1\ell_1ℓ1​ must pass through the origin and contain direction (1,1,1)(1,1,1)(1,1,1). Hence its normal vector n⃗\vec nn satisfies n⃗⋅(1,1,1)=0.\vec n\cdot (1,1,1)=0.n⋅(1,1,1)=0. Therefore plane equation is n⃗⋅r⃗=0,\vec n\cdot \vec r=0,n⋅r=0, where $\vec n\perp (1,1,1).$$

  3. Distance from line ℓ2\ell_2ℓ2​ to plane HHH

    A point on ℓ2\ell_2ℓ2​ is r⃗=(0,1,−1)+μ(1,0,1).\vec r=(0,1,-1)+\mu(1,0,1).r=(0,1,−1)+μ(1,0,1). Its signed value in the plane equation is n⃗⋅r⃗=n⃗⋅(0,1,−1)+μ n⃗⋅(1,0,1).\vec n\cdot \vec r=\vec n\cdot(0,1,-1)+\mu\,\vec n\cdot(1,0,1).n⋅r=n⋅(0,1,−1)+μn⋅(1,0,1).

    The minimum distance from points of ℓ2\ell_2ℓ2​ to plane HHH is zero if the line intersects the plane, i.e. if n⃗⋅(1,0,1)≠0\vec n\cdot(1,0,1)\neq 0n⋅(1,0,1)=0.

    For a positive distance, ℓ2\ell_2ℓ2​ must be parallel to the plane, so we need n⃗⋅(1,0,1)=0.\vec n\cdot(1,0,1)=0.n⋅(1,0,1)=0.

    Thus n⃗\vec nn must be perpendicular to both (1,1,1)(1,1,1)(1,1,1) and (1,0,1)(1,0,1)(1,0,1). Hence n⃗∥(1,1,1)×(1,0,1).\vec n \parallel (1,1,1)\times(1,0,1).n∥(1,1,1)×(1,0,1).

    Compute:

    (1,1,1)×(1,0,1)=∣i^j^k^111101∣=(1,0,−1).(1,1,1)\times(1,0,1) =\begin{vmatrix} \hat i&\hat j&\hat k\\ 1&1&1\\ 1&0&1 \end{vmatrix} =(1,0,-1).(1,1,1)×(1,0,1)=​i^11​j^​10​k^11​​=(1,0,−1).

    So H0H_0H0​ has normal vector (1,0,−1)(1,0,-1)(1,0,−1), hence equation x−z=0.x-z=0.x−z=0.

  4. Compute d(H0)d(H_0)d(H0​)

    Since ℓ2\ell_2ℓ2​ is parallel to H0H_0H0​, distance from the line to plane equals distance of any point on ℓ2\ell_2ℓ2​ from plane. Take point (0,1,−1)(0,1,-1)(0,1,−1).

    Distance from (x0,y0,z0)(x_0,y_0,z_0)(x0​,y0​,z0​) to plane x−z=0x-z=0x−z=0 is

    Therefore d(H0)=∣0−(−1)∣2=12.d(H_0)=\frac{|0-(-1)|}{\sqrt2}=\frac{1}{\sqrt2}.d(H0​)=2​∣0−(−1)∣​=2​1​.

    So (P)→(5).(P)\to(5).(P)→(5).

  5. Distance of point (0,1,2)(0,1,2)(0,1,2) from H0H_0H0​

    dist((0,1,2),H0)=∣0−2∣2=22=2.\text{dist}((0,1,2),H_0)=\frac{|0-2|}{\sqrt2}=\frac{2}{\sqrt2}=\sqrt2.dist((0,1,2),H0​)=2​∣0−2∣​=2​2​=2​.

    So (Q)→(4).(Q)\to(4).(Q)→(4).

  6. Distance of origin from H0H_0H0​

    Since plane H0:x−z=0H_0: x-z=0H0​:x−z=0 passes through origin, dist((0,0,0),H0)=0.\text{dist}((0,0,0),H_0)=0.dist((0,0,0),H0​)=0.

    So (R)→(3).(R)\to(3).(R)→(3).

  7. Intersection of planes y=zy=zy=z, x=1x=1x=1, and H0H_0H0​

    From H0H_0H0​, we have x=z.x=z.x=z. Also given x=1,y=z.x=1,\qquad y=z.x=1,y=z. Hence x=1, z=1, y=1.x=1,\ z=1,\ y=1.x=1, z=1, y=1. So intersection point is (1,1,1).(1,1,1).(1,1,1).

    Distance of origin from this point is 12+12+12=3.\sqrt{1^2+1^2+1^2}=\sqrt3.12+12+12​=3​.

    Thus (S)→(1).(S)\to(1).(S)→(1).

  8. Final matching

    (P)→(5),(Q)→(4),(R)→(3),(S)→(1).(P)\to(5),\quad (Q)\to(4),\quad (R)\to(3),\quad (S)\to(1).(P)→(5),(Q)→(4),(R)→(3),(S)→(1).

    This corresponds to Option B.

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