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3D Geometry question

2022 · Shift 1 · Q29
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  5. /2022 · Shift 1 · Q29

3D Geometry question

2022 · Shift 1 · Q29

JEE AdvancedMathematics3D GeometryMultiple correct+4 / −2
Let P1P_{1}P1​ and P2P_{2}P2​ be two planes given by P1:10x+15y+12z−60=0P2:−2x+5y+4z−20=0\begin{aligned} &P_{1}: 10 x+15 y+12 z-60=0 \\\\ &P_{2}:-2 x+5 y+4 z-20=0 \end{aligned}​P1​:10x+15y+12z−60=0P2​:−2x+5y+4z−20=0​ Which of the following straight lines can be an edge of some tetrahedron whose two faces lie on P1P_{1}P1​ and P2P_{2}P2​ ?
  1. A
    x−10=y−10=z−15\frac{x-1}{0}=\frac{y-1}{0}=\frac{z-1}{5}0x−1​=0y−1​=5z−1​
  2. B
    x−6−5=y2=z3\frac{x-6}{-5}=\frac{y}{2}=\frac{z}{3}−5x−6​=2y​=3z​
  3. C
    x−2=y−45=z4\frac{x}{-2}=\frac{y-4}{5}=\frac{z}{4}−2x​=5y−4​=4z​
  4. D
    x1=y−4−2=z3\frac{x}{1}=\frac{y-4}{-2}=\frac{z}{3}1x​=−2y−4​=3z​
View written solutionFree

Correct answer: NONE OF A, B, C, D

We need the lines which can be an edge of some tetrahedron whose two faces lie on P1P_1P1​ and P2P_2P2​.

1. Key geometric idea

If two faces of a tetrahedron lie on planes P1P_1P1​ and P2P_2P2​, then these two faces intersect along an edge of the tetrahedron.

So the required line must be the line of intersection of the two planes P1P_1P1​ and P2P_2P2​.

Thus, among the given options, we must find which lines coincide with P1∩P2.P_1 \cap P_2.P1​∩P2​.


2. Find the intersection line of P1P_1P1​ and P2P_2P2​

The planes are: P1:10x+15y+12z−60=0P_1: 10x+15y+12z-60=0P1​:10x+15y+12z−60=0 P2:−2x+5y+4z−20=0P_2: -2x+5y+4z-20=0P2​:−2x+5y+4z−20=0

So, 10x+15y+12z=60...(1)10x+15y+12z=60 \quad ...(1)10x+15y+12z=60...(1) −2x+5y+4z=20...(2)-2x+5y+4z=20 \quad ...(2)−2x+5y+4z=20...(2)

Multiply (2) by 555: −10x+25y+20z=100-10x+25y+20z=100−10x+25y+20z=100

Add to (1): 40y+32z=16040y+32z=16040y+32z=160 5y+4z=205y+4z=205y+4z=20 y=20−4z5...(3)y=\frac{20-4z}{5} \quad ...(3)y=520−4z​...(3)

Substitute in (2): −2x+5(20−4z5)+4z=20-2x+5\left(\frac{20-4z}{5}\right)+4z=20−2x+5(520−4z​)+4z=20 −2x+20−4z+4z=20-2x+20-4z+4z=20−2x+20−4z+4z=20 −2x=0-2x=0−2x=0 x=0.x=0.x=0.

Hence the intersection line is given by x=0,5y+4z=20.x=0, \qquad 5y+4z=20.x=0,5y+4z=20.

Let z=tz=tz=t. Then y=20−4t5=4−4t5.y=\frac{20-4t}{5}=4-\frac{4t}{5}.y=520−4t​=4−54t​.

A convenient parametric form is obtained by taking t=5λt=5\lambdat=5λ: x=0,y=4−4λ,z=5λ.x=0, \qquad y=4-4\lambda, \qquad z=5\lambda.x=0,y=4−4λ,z=5λ.

So the line is (x,y,z)=(0,4,0)+λ(0,−4,5).(x,y,z)=(0,4,0)+\lambda(0,-4,5).(x,y,z)=(0,4,0)+λ(0,−4,5).

Any equivalent direction vector is acceptable, e.g. (0,4,−5)(0,4,-5)(0,4,−5).


3. Check each option

Option A

x−10=y−10=z−15\frac{x-1}{0}=\frac{y-1}{0}=\frac{z-1}{5}0x−1​=0y−1​=5z−1​ This means x=1,y=1,z=1+5t.x=1,\quad y=1,\quad z=1+5t.x=1,y=1,z=1+5t. So direction vector is (0,0,5)(0,0,5)(0,0,5).

This line is parallel to the zzz-axis and does not satisfy x=0x=0x=0 or 5y+4z=205y+4z=205y+4z=20 for all points. Hence it is not the intersection line.

So A is incorrect.


Option B

x−6−5=y2=z3=t\frac{x-6}{-5}=\frac{y}{2}=\frac{z}{3}=t−5x−6​=2y​=3z​=t So x=6−5t,y=2t,z=3t.x=6-5t,\quad y=2t,\quad z=3t.x=6−5t,y=2t,z=3t.

Check if every point lies on both planes.

For P1P_1P1​: 10x+15y+12z−60=10(6−5t)+15(2t)+12(3t)−6010x+15y+12z-60=10(6-5t)+15(2t)+12(3t)-6010x+15y+12z−60=10(6−5t)+15(2t)+12(3t)−60 =60−50t+30t+36t−60=16t.=60-50t+30t+36t-60=16t.=60−50t+30t+36t−60=16t. This is not identically zero.

Hence line is not in P1P_1P1​; so it cannot be the common edge. Thus B is incorrect.


Option C

x−2=y−45=z4=t\frac{x}{-2}=\frac{y-4}{5}=\frac{z}{4}=t−2x​=5y−4​=4z​=t So x=−2t,y=4+5t,z=4t.x=-2t,\quad y=4+5t,\quad z=4t.x=−2t,y=4+5t,z=4t.

For P1P_1P1​: 10x+15y+12z−60=10(−2t)+15(4+5t)+12(4t)−6010x+15y+12z-60=10(-2t)+15(4+5t)+12(4t)-6010x+15y+12z−60=10(−2t)+15(4+5t)+12(4t)−60 =−20t+60+75t+48t−60=103t,=-20t+60+75t+48t-60=103t,=−20t+60+75t+48t−60=103t, not identically zero.

So C is incorrect.


Option D

x1=y−4−2=z3=t\frac{x}{1}=\frac{y-4}{-2}=\frac{z}{3}=t1x​=−2y−4​=3z​=t So x=t,y=4−2t,z=3t.x=t,\quad y=4-2t,\quad z=3t.x=t,y=4−2t,z=3t.

For P1P_1P1​: 10x+15y+12z−60=10t+15(4−2t)+12(3t)−6010x+15y+12z-60=10t+15(4-2t)+12(3t)-6010x+15y+12z−60=10t+15(4−2t)+12(3t)−60 =10t+60−30t+36t−60=16t,=10t+60-30t+36t-60=16t,=10t+60−30t+36t−60=16t, not identically zero.

So D is incorrect.


4. Conclusion

None of the given lines equals the line of intersection of P1P_1P1​ and P2P_2P2​.

Therefore, none of the options can be the edge common to the two faces lying on P1P_1P1​ and P2P_2P2​.

So my derived answer is: None of A, B, C, D\boxed{\text{None of A, B, C, D}}None of A, B, C, D​


5. Comparison with stored answer

Stored correct answer: A, B, D

This does not match the geometric requirement. The common edge of the two faces must lie on both planes simultaneously, i.e. it must be their line of intersection. None of A, B, C, D satisfy both plane equations identically.

Hence I disagree with the stored answer.

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