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3D Geometry question

2023 · 13 Apr · Shift 2 · Q26
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  5. /2023 · 13 Apr · Shift 2 · Q26

3D Geometry question

2023 · 13 Apr · Shift 2 · Q26

JEE MainMathematics3D GeometryMCQ+4 / −1
The line, that is coplanar to the line x+3−3=y−11=z−55\frac{x+3}{-3}=\frac{y-1}{1}=\frac{z-5}{5}−3x+3​=1y−1​=5z−5​, is :
  1. A
    x+1−1=y−22=z−54\frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{4}−1x+1​=2y−2​=4z−5​
  2. B
    x+1−1=y−22=z−55\frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}−1x+1​=2y−2​=5z−5​
  3. C
    x−1−1=y−22=z−55\frac{x-1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}−1x−1​=2y−2​=5z−5​
  4. D
    x+11=y−22=z−55\frac{x+1}{1}=\frac{y-2}{2}=\frac{z-5}{5}1x+1​=2y−2​=5z−5​
View written solutionFree

Correct answer: B

  1. Given line

The line is

x+3−3=y−11=z−55.\frac{x+3}{-3}=\frac{y-1}{1}=\frac{z-5}{5}.−3x+3​=1y−1​=5z−5​.

Its point-direction form can be read as:

  • A point on the line: P(−3,1,5)P(-3,1,5)P(−3,1,5)
  • Direction vector: d⃗1=(−3,1,5)\vec d_1 = (-3,1,5)d1​=(−3,1,5)

So the given line is

r⃗=(−3,1,5)+λ(−3,1,5).\vec r = (-3,1,5) + \lambda(-3,1,5).r=(−3,1,5)+λ(−3,1,5).
  1. Condition for two lines to be coplanar

Two lines are coplanar if the scalar triple product of:

  • direction vector of first line,
  • direction vector of second line,
  • vector joining one point on first line to one point on second line,

is zero.

That is,

[d⃗1,d⃗2,P1P2→]=0.[\vec d_1,\vec d_2,\overrightarrow{P_1P_2}] = 0.[d1​,d2​,P1​P2​​]=0.
  1. Check each option

We take P1=(−3,1,5)P_1=(-3,1,5)P1​=(−3,1,5) and d⃗1=(−3,1,5)\vec d_1=(-3,1,5)d1​=(−3,1,5).

Option A

x+1−1=y−22=z−54\frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{4}−1x+1​=2y−2​=4z−5​

Point: QA=(−1,2,5)Q_A=(-1,2,5)QA​=(−1,2,5)

Direction vector:

d⃗A=(−1,2,4)\vec d_A = (-1,2,4)dA​=(−1,2,4)

Vector joining points:

P1QA→=(−1+3,2−1,5−5)=(2,1,0)\overrightarrow{P_1Q_A} = (-1+3,2-1,5-5)=(2,1,0)P1​QA​​=(−1+3,2−1,5−5)=(2,1,0)

Now compute scalar triple product:

∣−315−124210∣\begin{vmatrix} -3 & 1 & 5 \\ -1 & 2 & 4 \\ 2 & 1 & 0 \end{vmatrix}​−3−12​121​540​​

Expanding:

=−3∣2410∣−1∣−1420∣+5∣−1221∣= -3\begin{vmatrix}2 & 4 \\ 1 & 0\end{vmatrix} -1\begin{vmatrix}-1 & 4 \\ 2 & 0\end{vmatrix} +5\begin{vmatrix}-1 & 2 \\ 2 & 1\end{vmatrix}=−3​21​40​​−1​−12​40​​+5​−12​21​​ =−3(2⋅0−4⋅1)−(−1⋅0−4⋅2)+5((−1)(1)−2⋅2)= -3(2\cdot 0 - 4\cdot 1) - ( -1\cdot 0 - 4\cdot 2 ) + 5((-1)(1)-2\cdot 2)=−3(2⋅0−4⋅1)−(−1⋅0−4⋅2)+5((−1)(1)−2⋅2) =−3(−4)−(−8)+5(−1−4)=12+8−25=−5≠0= -3(-4) - (-8) + 5(-1-4) =12+8-25=-5 \neq 0=−3(−4)−(−8)+5(−1−4)=12+8−25=−5=0

So A is not coplanar.


Option B

x+1−1=y−22=z−55\frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}−1x+1​=2y−2​=5z−5​

Point: QB=(−1,2,5)Q_B=(-1,2,5)QB​=(−1,2,5)

Direction vector:

d⃗B=(−1,2,5)\vec d_B = (-1,2,5)dB​=(−1,2,5)

Vector joining points:

P1QB→=(2,1,0)\overrightarrow{P_1Q_B}=(2,1,0)P1​QB​​=(2,1,0)

Scalar triple product:

∣−315−125210∣\begin{vmatrix} -3 & 1 & 5 \\ -1 & 2 & 5 \\ 2 & 1 & 0 \end{vmatrix}​−3−12​121​550​​

Expanding:

=−3∣2510∣−1∣−1520∣+5∣−1221∣= -3\begin{vmatrix}2 & 5 \\ 1 & 0\end{vmatrix} -1\begin{vmatrix}-1 & 5 \\ 2 & 0\end{vmatrix} +5\begin{vmatrix}-1 & 2 \\ 2 & 1\end{vmatrix}=−3​21​50​​−1​−12​50​​+5​−12​21​​ =−3(2⋅0−5⋅1)−((−1)⋅0−5⋅2)+5((−1)(1)−2⋅2)= -3(2\cdot 0-5\cdot 1) -((-1)\cdot 0-5\cdot 2)+5((-1)(1)-2\cdot 2)=−3(2⋅0−5⋅1)−((−1)⋅0−5⋅2)+5((−1)(1)−2⋅2) =−3(−5)−(−10)+5(−5)=15+10−25=0= -3(-5)-(-10)+5(-5)=15+10-25=0=−3(−5)−(−10)+5(−5)=15+10−25=0

So B is coplanar.


Option C

x−1−1=y−22=z−55\frac{x-1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}−1x−1​=2y−2​=5z−5​

Point: QC=(1,2,5)Q_C=(1,2,5)QC​=(1,2,5)

Direction vector:

d⃗C=(−1,2,5)\vec d_C=(-1,2,5)dC​=(−1,2,5)

Vector joining points:

P1QC→=(1+3,2−1,5−5)=(4,1,0)\overrightarrow{P_1Q_C}=(1+3,2-1,5-5)=(4,1,0)P1​QC​​=(1+3,2−1,5−5)=(4,1,0)

Scalar triple product:

∣−315−125410∣\begin{vmatrix} -3 & 1 & 5 \\ -1 & 2 & 5 \\ 4 & 1 & 0 \end{vmatrix}​−3−14​121​550​​

Expanding:

=−3∣2510∣−1∣−1540∣+5∣−1241∣= -3\begin{vmatrix}2 & 5 \\ 1 & 0\end{vmatrix} -1\begin{vmatrix}-1 & 5 \\ 4 & 0\end{vmatrix} +5\begin{vmatrix}-1 & 2 \\ 4 & 1\end{vmatrix}=−3​21​50​​−1​−14​50​​+5​−14​21​​ =−3(−5)−((−1)⋅0−5⋅4)+5((−1)(1)−2⋅4)= -3(-5)-((-1)\cdot 0-5\cdot 4)+5((-1)(1)-2\cdot 4)=−3(−5)−((−1)⋅0−5⋅4)+5((−1)(1)−2⋅4) =15−(−20)+5(−9)=15+20−45=−10≠0=15-(-20)+5(-9)=15+20-45=-10\neq 0=15−(−20)+5(−9)=15+20−45=−10=0

So C is not coplanar.


Option D

x+11=y−22=z−55\frac{x+1}{1}=\frac{y-2}{2}=\frac{z-5}{5}1x+1​=2y−2​=5z−5​

Point: QD=(−1,2,5)Q_D=(-1,2,5)QD​=(−1,2,5)

Direction vector:

d⃗D=(1,2,5)\vec d_D=(1,2,5)dD​=(1,2,5)

Vector joining points:

P1QD→=(2,1,0)\overrightarrow{P_1Q_D}=(2,1,0)P1​QD​​=(2,1,0)

Scalar triple product:

∣−315125210∣\begin{vmatrix} -3 & 1 & 5 \\ 1 & 2 & 5 \\ 2 & 1 & 0 \end{vmatrix}​−312​121​550​​

Expanding:

=−3∣2510∣−1∣1520∣+5∣1221∣= -3\begin{vmatrix}2 & 5 \\ 1 & 0\end{vmatrix} -1\begin{vmatrix}1 & 5 \\ 2 & 0\end{vmatrix} +5\begin{vmatrix}1 & 2 \\ 2 & 1\end{vmatrix}=−3​21​50​​−1​12​50​​+5​12​21​​ =−3(−5)−(1⋅0−5⋅2)+5(1⋅1−2⋅2)= -3(-5) - (1\cdot 0-5\cdot 2)+5(1\cdot 1-2\cdot 2)=−3(−5)−(1⋅0−5⋅2)+5(1⋅1−2⋅2) =15−(−10)+5(−3)=15+10−15=10≠0=15-(-10)+5(-3)=15+10-15=10\neq 0=15−(−10)+5(−3)=15+10−15=10=0

So D is not coplanar.


  1. Conclusion

Only Option B satisfies the coplanarity condition.

Therefore, the required line is

x+1−1=y−22=z−55\boxed{\frac{x+1}{-1}=\frac{y-2}{2}=\frac{z-5}{5}}−1x+1​=2y−2​=5z−5​​
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