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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

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 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

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

If the vector equation of a line is  find its caryesian equation.
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Gaurav Kumar answered
r = -2i + 3j + 7k + λ( -i - 2j - 3k)
xi + yj + zk = (-2-λ)i + (3-2λ)j + (7-3λ)k
Equating the terms, we get
x = -2-λ     y = 3-2λ   z = 7-3λ
(x+2)/(-1) = λ,   (y-3)/(-2) = λ,    (z-7)/(-3) = λ
(x+2)/(1) =  (y-3)/(2) =  (z-7)/(3) 

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.

If a line in the ZX-plane makes an angle 60o with Z-axis, the direction cosines of this line are:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Saumya Gupta answered
The angle between the line and z axis is 60 and the line in ZX plane so the angle between line and X axis is (90-60)=30 and line is perpendicular to Y axis so the angle between line and Y axis is 90. So direction cosine of line is cos30,cos90,cos60 =√3/2,0,1/2.

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: 

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.

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.

Direction: In the following questions, A statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as
Assertion (A): P is a point on the line segment joining the points (3, 2, –1) and (6, –4, –2). If x coordinate of P is 5, then its y coordinate is –2.
Reason (R): The two lines x = ay + b, z = cy + d and x = a’y + b’, z = c’y + d’ will be perpendicular, iff aa’ + bb’ + cc’ = 0.
  • a)
    Both A and R are true and R is the correct explanation of A
  • b)
    Both A and R are true but R is NOT the correct explanation of A
  • c)
    A is true but R is false
  • d)
    A is false but R is True
Correct answer is option 'C'. Can you explain this answer?

Shalini Bose answered
Assertion (A): P is a point on the line segment joining the points (3, 2, –1) and (6, –4, –2). If x coordinate of P is 5, then its y coordinate is –2.

Reason (R): The two lines x = ay b, z = cy d and x = a’y b’, z = c’y d’ will be perpendicular, iff aa’ bb’ cc’ = 0.

The given assertion relates to the coordinates of a point P on a line segment joining two given points. The reason provided relates to the condition for two lines to be perpendicular.

Explanation:

To understand this assertion and reason, let's first find the equation of the line segment joining the points (3, 2, –1) and (6, –4, –2).

The equation of a line segment joining two points (x₁, y₁, z₁) and (x₂, y₂, z₂) can be written as:
(x - x₁)/(x₂ - x₁) = (y - y₁)/(y₂ - y₁) = (z - z₁)/(z₂ - z₁)

Plugging in the given values, we have:
(x - 3)/(6 - 3) = (y - 2)/(-4 - 2) = (z + 1)/(-2 - (-1))

Simplifying, we get:
(x - 3)/3 = (y - 2)/(-6) = (z + 1)/(-1)

Now, let's consider the x-coordinate of P as 5. Substituting this value into the equation, we get:
(5 - 3)/3 = (y - 2)/(-6) = (z + 1)/(-1)

2/3 = (y - 2)/(-6) = (z + 1)/(-1)

Simplifying further, we get:
(y - 2)/(-6) = (z + 1)/(-1)

Cross-multiplying, we get:
-6(y - 2) = -1(z + 1)

Expanding, we get:
-6y + 12 = -z - 1

Rearranging, we get:
z = -6y + 13

Comparing this equation with the given equations:
x = ay + b
z = cy + d

We can see that a = 0, b = 0, c = -6, and d = 13.

Now, let's consider the given reason. According to the reason, the two lines x = ay + b, z = cy + d and x = a'y + b', z = c'y + d' will be perpendicular if aa' + bb' + cc' = 0.

Plugging in the values, we have:
(0)(0) + (0)(0) + (-6)(0) = 0

Therefore, aa' + bb' + cc' = 0, which means the two lines are perpendicular.

Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

Hence, the correct answer is option C.

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