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3D Geometry question

2021 · 25 Feb · Shift 1 · Q25
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  5. /2021 · 25 Feb · Shift 1 · Q25

3D Geometry question

2021 · 25 Feb · Shift 1 · Q25

JEE MainMathematics3D GeometryMCQ+4 / −1
The equation of the line through the point (0, 1, 2) and perpendicular to the line x−12=y+13=z−1−2{{x - 1} \over 2} = {{y + 1} \over 3} = {{z - 1} \over { - 2}}2x−1​=3y+1​=−2z−1​ is :
  1. A
    x3=y−1−4=z−23{x \over 3} = {{y - 1} \over { - 4}} = {{z - 2} \over 3}3x​=−4y−1​=3z−2​
  2. B
    x3=y−14=z−2−3{x \over 3} = {{y - 1} \over 4} = {{z - 2} \over { - 3}}3x​=4y−1​=−3z−2​
  3. C
    x−3=y−14=z−23{x \over { - 3}} = {{y - 1} \over 4} = {{z - 2} \over 3}−3x​=4y−1​=3z−2​
  4. D
    x3=y−14=z−23{x \over 3} = {{y - 1} \over 4} = {{z - 2} \over 3}3x​=4y−1​=3z−2​
View written solutionFree

Correct answer: C

  1. Given line and its direction vector

    The line x−12=y+13=z−1−2\frac{x-1}{2}=\frac{y+1}{3}=\frac{z-1}{-2}2x−1​=3y+1​=−2z−1​ has direction ratios d⃗=(2,3,−2).\vec d=(2,3,-2).d=(2,3,−2).

  2. Required line

    We need a line passing through the point (0,1,2)(0,1,2)(0,1,2) and perpendicular to the given line.

    So if the required line has direction ratios (l,m,n)(l,m,n)(l,m,n), then they must satisfy 2l+3m−2n=0.2l+3m-2n=0.2l+3m−2n=0.

  3. Check each option

    Since all options pass through (0,1,2)(0,1,2)(0,1,2), we only need to test their direction ratios.


    Option A: x3=y−1−4=z−23\frac{x}{3}=\frac{y-1}{-4}=\frac{z-2}{3}3x​=−4y−1​=3z−2​ Direction ratios: (3,−4,3)(3,-4,3)(3,−4,3)

    Dot product with (2,3,−2)(2,3,-2)(2,3,−2): 2(3)+3(−4)−2(3)=6−12−6=−12≠02(3)+3(-4)-2(3)=6-12-6=-12\neq 02(3)+3(−4)−2(3)=6−12−6=−12=0 So, A is not perpendicular.


    Option B: x3=y−14=z−2−3\frac{x}{3}=\frac{y-1}{4}=\frac{z-2}{-3}3x​=4y−1​=−3z−2​ Direction ratios: (3,4,−3)(3,4,-3)(3,4,−3)

    Dot product: 2(3)+3(4)−2(−3)=6+12+6=24≠02(3)+3(4)-2(-3)=6+12+6=24\neq 02(3)+3(4)−2(−3)=6+12+6=24=0 So, B is not perpendicular.


    Option C: x−3=y−14=z−23\frac{x}{-3}=\frac{y-1}{4}=\frac{z-2}{3}−3x​=4y−1​=3z−2​ Direction ratios: (−3,4,3)(-3,4,3)(−3,4,3)

    Dot product: 2(−3)+3(4)−2(3)=−6+12−6=02(-3)+3(4)-2(3)=-6+12-6=02(−3)+3(4)−2(3)=−6+12−6=0 So, C is perpendicular.


    Option D: x3=y−14=z−23\frac{x}{3}=\frac{y-1}{4}=\frac{z-2}{3}3x​=4y−1​=3z−2​ Direction ratios: (3,4,3)(3,4,3)(3,4,3)

    Dot product: 2(3)+3(4)−2(3)=6+12−6=12≠02(3)+3(4)-2(3)=6+12-6=12\neq 02(3)+3(4)−2(3)=6+12−6=12=0 So, D is not perpendicular.

  4. Conclusion

    The only line through (0,1,2)(0,1,2)(0,1,2) perpendicular to the given line is x−3=y−14=z−23\boxed{\frac{x}{-3}=\frac{y-1}{4}=\frac{z-2}{3}}−3x​=4y−1​=3z−2​​ which is Option C.

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