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All questions of Chapter 11 - Three Dimensional Geometry for JEE Exam

Find the equation of the set of points which are equidistant from the points (1, 2 , 3) and (3, 2, -1)​
a) x + 2z = 0
b) y + 2z = 0
c) x – 2z = 0
d) x – 2y = 0 
Correct answer is option 'C'. Can you explain this answer?

Aryan Khanna answered
Pt. A(1, 2 , 3)
Pt. B(3, 2, -1)
Let P(x,y,z)
So, AP = BP
((x-1)2 + (y-2)2 + (z-3)2)1/2 = ((x-3)2 + (y-2)2 + (z+1)2)1/2
(x-1)2 + (y-2)2 + (z-3)2) = (x-3)2 + (y-2)2 + (z+1)2
x2 +1 -2x + y2 + 4 - 4y + z2 + 9 – 6z = x2 +9 -6x + y2 + 4 - 4y + z2 + 1 + 2z
4x – 8z = 0
x – 2z = 0

The direction cosines of the line joining the points (2, -1, 8) and (-4, -3, 5) are:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Geetika Shah answered
Pt. A(2, -1, 8)
Pt. B(-4, -3, 5)
Direction Ratio DR of AB : ( -4-2 , -3+1 , 5-8 )
: (-6,-2,-3)
Direction cosine of AB : ( -6/(62+22+32)1/2 , -2/(62+22+32)1/2 , -3/(62+22+32)1/2)
: ( -6/7, -2/7, -3/7)
 

The distance of the point (2, 3, – 5) from the plane x + 2y – 2z = 9 is:​
  • a)
    2 units
  • b)
    3/2 units
  • c)
    3 units
  • d)
    10/3 units
Correct answer is option 'C'. Can you explain this answer?

Nikita Singh answered
 Length of perpendicular from (2,3,-5) to the plane x + 2y − 2z − 9 = 0.
= |(2 + 2×3 −2×(−5) − 9)|√12 + 22 + (−2)2
= |2 + 6 + 10 − 9|/√9
= 9/3
= 3 units.

The equation of the plane passing through the intersection of the planes  and and the point (1, 2, 1) is:​
  • a)
    18x+6y+14z-23=0
  • b)
    18x+7y+14z-46=0
  • c)
    9x+3y+7z-23=0
  • d)
    18x+7y+14z-38=0
Correct answer is option 'B'. Can you explain this answer?

Aryan Khanna answered
n1 = 2i + j + k
n2 = 2i + 3j - 4k
p1 = 4,   p2 = -6
r.(n1 + λn2) = p1 + λp2
=> r . [2i + j + k + λ(2i + 3j - 4k)] = 4 - 6λ
=> r . [ i(2 + 2λ) + j(1 + 3λ) + k(1 - 4k)] = 4 - 6λ
Taking r = xi + yj + zk
(2 + 2λ)x + (1 + 3λ)y + (1 - 4k)z = 4 - 6λ
(2x + y + - z - 4) + λ(2x + 3y - 4k + 6) = 0
Given points are (1,2,1) 
(2 + 2 - 1 - 4) + λ(2 + 6 - 4 + 6) = 0
-1 + λ(10) = 0
 λ = 1/10
Substitute  λ = 1/10, we get
18x + 7y + 14z - 46=0

The angle between the lines x = 2y = – 3z and – 4x = 6y = – z is:​
  • a)
  • b)
    cos-1(1/√3)
  • c)
    90°
  • d)
    180°
Correct answer is option 'C'. Can you explain this answer?

Sushil Kumar answered
x = 2y = -3z     -4x = 6y = -z
x/1 = y/(½) = z(-⅓)                   x/(-¼) = y/(⅙) = z/(-1)
Cosθ = [(a1a2 + b1b2 + c1c2)/(a1 + b1 + c1)½ * (a2 + b2 + c2)½]
Cosθ ={[(1*(-¼)) + (½)(⅙) + (-⅓)(-1)]/[(1)2 + (½)2 + (-⅓)2]1/2 * [(-¼)2 + (⅙)2 + (-1)2]1/2}
= {[(-¼ + 1/12  - ⅓)]/[2 + 1 - ⅔]1/2 * [ -½ + ⅓ -½]½}
Cosθ = 0
θ = 90deg

The equation of the plane passing through the line of intersection of the planes x-2y+3z+8=0 and 2x-7y+4z-3=0 and the point (3, 1, -2) is:​
  • a)
    6x-15y+12z+29=0
  • b)
    6x-15y+16z+29=0
  • c)
    6x-15y+12z+32=0
  • d)
    2x-5y+4z+9=0
Correct answer is option 'B'. Can you explain this answer?

Tejas Verma answered
(x - 2y + 3z + 8) + μ(2x - 7y + 4z - 3) = 0
i.e, (1 + 2μ)x - (2 + 7μ)y + (3 + 4μ)z + (8 - 3μ) = 0......(1)
 the required plane is passing through (3, 1, -2)
so, 3(1 + 2μ) - (1)(2 + 7μ) + (-2)(3 + 4μ)+ (8 - 3μ) = 0
3 + 6μ - 2 - 7μ -6 -8μ + 8 - 3μ = 0
by solving, μ = 1/4
putting μ in equation (1)
we get the required equation of plane as :- 6x - 15y + 16z + 29 = 0

If a line has the direction ratios -4, 18, -12 then what are its direction cosines?​
  • a)
    -2, 9, -6
  • b)
    -4, 18, -12
  • c)
    2/11, 9/11, 6/11
  • d)
    -2/11, 9/11, -6/11
Correct answer is option 'D'. Can you explain this answer?

Vikas Kapoor answered
DR of the line :  (-4, 18 -12)
DC of the line : (-4/k, 18/k, -12/k)
where k = ((42) + (182) + (12)2)1/2
= (16 + 324 + 144)1/2
= (484)1/2
= 22
So, DC : (-4/22, 18/22, -12/22)
: (-2/11 , 9/11 , -6/11)

The length of the perpendicular from the origin to the plane 3x + 2y – 6z = 21 is:​
  • a)
    3
  • b)
    14
  • c)
    21
  • d)
    7
Correct answer is option 'A'. Can you explain this answer?

Leelu Bhai answered
Given equation of plane is : 3x + 2y - 6z - 21= 0
the length of perpendicular from a given point
(x' , y', z') on a plane ax + by + cz + d = 0 is given as :-

d = modulus of [{ax' + by' + cz' + d}/{√(a² + b² + c)²}]

so, d = modulus of [{(3*0) + (2*0) + (-6*0) + (-21)}/{√(3² + 2² + (-6)²)}]

d= modulus of (-21/√49) = (-21/7) = 3 units
hence option A is correct....

The equation of plane through the intersection of planes (x+y+z =1) and (2x +3y – z+4) =0 is​
  • a)
    x(1 + 2k) + y(1 + 3k) + z(1 – k) + (-1 + 4k) = 0
  • b)
    x(1+2k)+y(1-3k)+z(1-k)+(-1+4k) = 0
  • c)
    x(1+2k) +y(1+3k)+z(1-k) +(-1 – 4k) = 0
  • d)
    x (1-2k) + y(1+3k) +z(1-k) +(-1+4k) = 0
Correct answer is option 'A'. Can you explain this answer?

Kritika Sarkar answered
To find the equation of the plane through the intersection of two given planes, we need to first determine the line of intersection and then find a normal vector for the plane.

1. Finding the line of intersection:
The two given planes are:
Plane 1: x + y + z = 1
Plane 2: 2x + 3y + z + 4 = 0

To find the line of intersection, we can set the equations of the two planes equal to each other:
x + y + z = 1
2x + 3y + z + 4 = 0

By subtracting the second equation from the first equation, we can eliminate z:
x + y + z - (2x + 3y + z + 4) = 1 - 0
-x - 2y - 4 = -1

Simplifying the equation, we get:
x + 2y = 3

This equation represents the line of intersection of the two planes.

2. Finding a normal vector for the plane:
Since the line of intersection lies on both planes, the normal vector of the required plane should be perpendicular to this line. Therefore, we can choose the direction ratios of the line, -1 and 2, as coefficients of the normal vector.

The normal vector of the required plane is given by the cross product of the direction ratios of the line of intersection:
n = (1, 2, 0) x (-1, 2, 0) = (0, 0, -4)

3. Writing the equation of the plane:
Now we have a point on the plane, (1, 0, 0), and a normal vector, (0, 0, -4). We can use the point-normal form of the equation of a plane to write the equation of the required plane.

The equation of the plane is given by:
0(x - 1) + 0(y - 0) + (-4)(z - 0) = 0

Simplifying the equation, we get:
-4z + 4 = 0
z = 1

Therefore, the equation of the plane through the intersection of the given planes is:
x + 2y - 4z + 4 = 0

Comparing this equation with the options provided, we see that option A is the correct answer:
x(1 2k) + y(1 3k) + z(1 k) + (-1 4k) = 0

Find the direction cosines of the x axis.​
  • a)
    1, 0, 0
  • b)
    0, 0, 0
  • c)
    0, 1, 0
  • d)
    0, 0, 1
Correct answer is option 'A'. Can you explain this answer?

To find Direction Cosines of X-axis.
Take any two points on X-axis : A(a,0,0) & B(b,0,0)
DR of AB : (b-a,0,0)
DC of AB : ((b-a)/(((b-a)2 + 0 + 0)1/2), 0, 0)
: ((b-a)/(b-a) , 0 , 0)
: (1,0,0)

The equation of the plane, which is at a distance of 5 unit from the origin and has  as a normal vector, is:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Preeti Iyer answered
x = 3i - 2j - 6k
|x| = ((3)2 + (2)2 + (6)2)
|x| = (49)½ 
|x| = 7
x = x/|x|
= (3i - 2j - 6k)/7
The required equation of plane is r.x = d
⇒  r.(3i - 2j - 6k)/7 = 5
⇒  r.(3i - 2j - 6k) = 35

For which value of a lines  and  are perpendicular?
  • a)
    11/70
  • b)
    5
  • c)
    1
  • d)
    70/11
Correct answer is option 'B'. Can you explain this answer?

Tejas Verma answered
(x-1)/(-3) = (y-2)/(2p/7) = (z-3)/2 
(x-1)(-3p/7) = (y - 5)/1 = (z - 6)/(-5)
The direction ratio of the line are -3, 2p/7, -2 and (-3p)/7, 1, -5
Two lines with direction ratios, a1, b1, c1 and a2, b2, c2 are perpendicular to each other if a1a2 + b1b2 + c1c2 = 0
Therefore, (-3)(-3p/7) + (2p/7)(1) + 2(-5) = 0
(9p/7) + (2p/7) = 10
11p = 70
p = 70/1

The equation of the plane passing through the point (3, – 3, 1) and perpendicular to the line joining the points (3, 4, – 1) and (2, – 1, 5) is:​
  • a)
    – x – 5y + 6z + 18 = 0
  • b)
    x – 5y + 6z + 18 = 0
  • c)
    x + 5y – 6z + 18 = 0
  • d)
    – x – 5y – 6z + 18 = 0
Correct answer is option 'C'. Can you explain this answer?

Preeti Iyer answered
The equation of the plane passing through the point (3, – 3, 1) is:
a(x – 3) + b(y + 3) + c(z – 1) = 0 and the direction ratios of the line joining the points
(3, 4, – 1) and (2, – 1, 5) is 2 – 3, – 1 – 4, 5 + 1, i.e., – 1, – 5, 6.
Since the plane is perpendicular to the line whose direction ratios are – 1, – 5, 6, therefore, direction ratios of the normal to the plane is – 1, – 5, 6.
So, required equation of plane is: – 1(x – 3) – 5(y + 3) + 6(z – 1) = 0
i.e., x +  5y – 6z + 18 = 0.

In the following case, determine whether the given planes are parallel orperpendicular, and in case they are neither, find the angles between them. 2x + y + 3z – 2 = 0 and x – 2y + 5 = 0
  • a)
    The planes are parallel
  • b)
    The planes are at 45
  • c)
    The planes are at 55
  • d)
    The planes are perpendicular
Correct answer is option 'D'. Can you explain this answer?

Understanding the Given Planes
To determine the relationship between the two planes, we need to first rewrite their equations in a standard format and extract the normal vectors.
Plane Equations
1. First Plane:
The equation is given as 2x + y + 3z - 2 = 0.
The normal vector (N1) can be derived from the coefficients:
N1 = (2, 1, 3).
2. Second Plane:
The equation is x - 2y + 5 = 0, which can be rewritten as x - 2y + 0z + 5 = 0.
The normal vector (N2) can be derived as:
N2 = (1, -2, 0).
Checking for Parallelism
- Two planes are parallel if their normal vectors are scalar multiples of each other.
- Here, N1 = (2, 1, 3) and N2 = (1, -2, 0) are not scalar multiples.
- Thus, the planes are not parallel.
Checking for Perpendicularity
- Two planes are perpendicular if the dot product of their normal vectors equals zero.
- We calculate the dot product:
N1 • N2 = (2)(1) + (1)(-2) + (3)(0) = 2 - 2 + 0 = 0.
- Since the dot product is zero, the planes are perpendicular.
Conclusion
The given planes are neither parallel nor at a specified angle; they are indeed perpendicular to each other. Thus, the correct answer is option 'D'.

Find the equations of the planes that passes through three points (1, 1, 0), (1, 2, 1), (– 2, 2, – 1)
  • a)
    2x + 3y – 7z = 5
  • b)
    2x + 5y – 3z = 5
  • c)
    3x + 3y – 3z = 5
  • d)
    2x + 3y – 3z = 5
Correct answer is option 'D'. Can you explain this answer?

Anand Khanna answered
In cartesian co-ordinate system :
Equation of a plane passing through three non collinear
Points (x1, y1, z1) , (x2, y2, z2) and (x3, y3, z3) is given by :



Therefore, the equations of the planes that passes through three points (1,1,0), (1,2,1),  (-2,2,-1) is given by :



⇒ (x-1)(-2) - (y-1) (3) + 3z = 0
⇒ 2x+3y - 3z = 5

The angle between two lines whose direction ratios are 1,2,1 and 2,-3,4 is:​
  • a)
    30°
  • b)
    60°
  • c)
    90°
  • d)
    45°
Correct answer is option 'C'. Can you explain this answer?

O Kaif Sid answered
Cosx =( i+2j+k).(2i-3j+4k)/ √1+4+1 × √4+9+16
cosx= 2-6+4/√1+4+1 × √4+9+16
cosx = 0/√1+4+1 × √4+9+16
cosx = 0
therfore
x = 90

Find the distance of the point (0, 0, 0) from the plane 3x – 4y + 12 z = 3
  • a)
    9/13
  • b)
    7/13
  • c)
    5/13
  • d)
    3/13
Correct answer is option 'D'. Can you explain this answer?

Krish Ghoshal answered
As we know that the length of the perpendicular from point 
P(x1,y1,z1) from the plane a1x+b1y+c1z+d1 = 0 is given by: 

Find the direction cosines of a line which makes equal angles with all three the coordinate axes.​
  • a)
    ± 1/√2, ± 1/√2, ± 1/√2
  • b)
    1/√3, 1/√3, 1/√3
  • c)
    1/√2, 1/√2, 1/√2
  • d)
    ± 1/√3, ± 1/√3, ± 1/√3
Correct answer is option 'D'. Can you explain this answer?

Sakshi Shah answered
Understanding Direction Cosines
Direction cosines are the cosines of the angles that a line makes with the coordinate axes. For a line to make equal angles with all three axes, the direction cosines must be equal in magnitude.
Equal Angles with Axes
If a line makes equal angles with the x, y, and z axes, then we can denote the direction cosines as:
- l = cos(α)
- m = cos(β)
- n = cos(γ)
Where α, β, and γ are the angles with the x, y, and z axes, respectively.
Since the angles are equal, we have:
- l = m = n
Magnitude Condition
The sum of the squares of the direction cosines must equal 1:
- l² + m² + n² = 1
Substituting l = m = n gives:
- 3l² = 1
- l² = 1/3
- l = ±1/√3
Thus, the direction cosines can be expressed as:
- l = ±1/√3
- m = ±1/√3
- n = ±1/√3
Conclusion
The direction cosines that make equal angles with the coordinate axes are:
- ±1/√3, ±1/√3, ±1/√3
This corresponds to option 'D':
- Direction cosines: ±1/√3, ±1/√3, ±1/√3
Therefore, the correct answer is indeed option 'D'.

The direction cosines of the line equally inclined with the axes, are:​
  • a)
    1, 1, 1,
  • b)
    0, 0, 0
  • c)
    1/3, 1/3, 1/3
  • d)
    1/√3, 1/√3, 1/√3
Correct answer is option 'D'. Can you explain this answer?

Geetika Mehta answered
Direction cosines are the ratios of the direction ratios of a line. In other words, they represent the cosines of the angles that the line makes with the positive x, y, and z axes.

Let's consider a line equally inclined with the axes. This means that the line makes equal angles with each axis.

To find the direction cosines of this line, we need to determine the cosines of the angles it makes with the x, y, and z axes.

Let's assume that the line makes an angle of θ with each axis. Since the line is equally inclined with the axes, all the angles are equal.

Let's calculate the direction cosine along the x-axis:

cos(θ) = adjacent side / hypotenuse
cos(θ) = 1 / hypotenuse
hypotenuse = 1 / cos(θ)

Similarly, the direction cosine along the y-axis and z-axis would also be 1 / cos(θ).

Therefore, the direction cosines of the line equally inclined with the axes are (1 / cos(θ), 1 / cos(θ), 1 / cos(θ)).

Since the line is equally inclined with the axes, the angle θ is the same for all three direction cosines.

Hence, the correct answer is option D: 1/3, 1/3, 1/3.

Angle between skew lines is
  • a)
    the angle between two non intersecting lines drawn from any point parallel to each of the skew lines
  • b)
    the angle between two intersecting lines drawn from any point perpendicular to each of the skew lines
  • c)
    the angle between two non intersecting lines drawn from any point anti – parallel to each of the skew lines
  • d)
    the angle between two intersecting lines drawn from any point parallel to each of the skew lines
Correct answer is option 'D'. Can you explain this answer?

Rhea Joshi answered
Understanding Skew Lines
Skew lines are non-parallel lines that do not intersect and exist in different planes. To find the angle between skew lines, we need to adopt a specific approach.
Correct Option: D
The correct answer is option 'D': the angle between two intersecting lines drawn from any point parallel to each of the skew lines. Here’s why this is the case:
Angle Definition
- The angle between two lines is typically defined using the lines' directions or their projections onto a common plane.
- For skew lines, we cannot directly measure the angle as they do not meet.
Using Parallel Lines to Determine Angle
- By drawing lines from a common point that are parallel to each of the skew lines, we create a scenario where the angle between these lines can be calculated conveniently.
- These auxiliary lines will intersect, allowing us to measure the angle formed.
Geometric Interpretation
- Imagine placing a point in space from which you can draw one line parallel to each skew line.
- The intersection of these two lines gives you a clear angle, which represents the angle between the original skew lines.
Why Other Options Are Incorrect
- Option A: Non-intersecting lines do not provide a defined angle measurement.
- Option B: Perpendicular lines do not relate to the original skew lines.
- Option C: Anti-parallel lines do not help in defining the angle between skew lines.
In conclusion, option 'D' is the correct choice as it efficiently utilizes the concept of parallel lines to determine the angle between skew lines through their intersection. This approach simplifies the understanding of angles in three-dimensional space.

Equation of a plane which is at a distance d from the origin and the direction cosines of the normal to the plane are l, m, n is.
  • a)
    lx – my + nz = d
  • b)
    – lx + my + nz = d
  • c)
    lx + my + nz = d
  • d)
    lx + my + nz = – d
Correct answer is option 'C'. Can you explain this answer?

In Cartesian co – ordinate system Equation of a plane which is at a distance d from the origin and the direction cosines of the normal to the plane are l, m, n is given by : lx + my + nz = d.

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