All questions of Vector Algebra and Three Dimensional Geometry for JEE Exam

If (2, 3, 5) is one end of a diameter of the sphere x2 + y2 + z2 – 6x – 12y – 2z + 20 = 0, then the cooordinates of the other end of the diameter are
  • a)
    (4, 3, 5)
  • b)
    (4, 3, – 3)
  • c)
    (4, 9, – 3)
  • d)
    (4, –3, 3).
Correct answer is option 'C'. Can you explain this answer?

Ram Mohith answered
The center for the givens sphere is (3,6,1) and one end of diameter is (2,3,5). So, we can find the other end using mid-point formula. Let the other end be (x,y,z) then,
3 = (2 + x)/2 => x = 4
6 = (3 + y)/2 => y = 9
1 = (5 + z)/2 => z = -3
Therefore, the other end of the diameter is (4,9,-3)

If the line,  lies in the plane, lx + my – z = 9,then l2 + m2 is equal to :
  • a)
    5
  • b)
    2
  • c)
    26
  • d)
    18
Correct answer is option 'B'. Can you explain this answer?

Rohit Jain answered
Line lies in the plane ⇒ (3, –2, –4) lie in the plane
⇒ 3ℓ – 2m + 4 = 9 or 3ℓ – 2m = 5 ..... (1)
Also, ℓ, m,–1 are dr's of line perpendicular to plane and 2, –1, 3 are dr's of line lying in the plane
⇒ 2ℓ – m – 3 = 0 or 2ℓ – m = 3 .....(2)
Solving (1) and (2) we get ℓ = 1 and m = –1
⇒ ℓ2 + m2 = 2

 are 3 vectors, such that  ,  then   is equal to
  • a)
    1
  • b)
    0
  • c)
    –7
  • d)
    7
Correct answer is option 'C'. Can you explain this answer?

Vikash Yadav answered
Square a+b+c=0
(a+b+c)^2 = a^2 + b^2 + c^2 + 2 a.b + 2 b.c + 2 c.a
0 = 1 + 4 + 9 + 2(a.b + b.c + c.a)
(a.b + b.c + c.a)= -14/2 = -7

If a line makes an angle of π /4 with the positive directions of each of x- axis and y- axis, then the angle that the line makes with the positive direction of the z-axis is
  • a)
    π /4
  • b)
    π /2
  • c)
    π /6
  • d)
    π /3
Correct answer is option 'B'. Can you explain this answer?

Let the angle of line makes with the positive direction of z-axis is a direction cosines of line with the +ve directions of x-axis, y-axis, and z-axis is l, m, n respectively.

Hence, angle with positive direction of the 

The value of k such that  lies in the plane 2x – 4y + z = 7, is
  • a)
    7
  • b)
    –7
  • c)
    no real value
  • d)
    4
Correct answer is option 'A'. Can you explain this answer?

Gaurav Kumar answered
As the line  lies in th e plan e 2x - 4 y +z= 7, the point (4, 2, k) through which line passes must also lie on the given plane and hence 2 × 4 – 4 × 2 + k = 7  ⇒ k = 7

Let the line  lie in the plane x + 3y – αz + β = 0. Then (α, β) equals
  • a)
    (–6, 7)
  • b)
    (5, –15)
  • c)
    (–5, 5)
  • d)
    (6, –17)
Correct answer is option 'A'. Can you explain this answer?

Sonu Kumar answered
Line lie in the plane and line passes through (2, 1,-2) then plane also passes through this point.
by satisfying find relation between alpha and beta.
and second direction ratio of line and plane are perpendicular then by dot products we get the value of alpha.

A line with positive direction cosines passes through the point P(2, –1, 2) and makes equal angles with the coordinate axes. The line meets the plane 2x + y + z = 9 at point Q. The length of the line segment PQ equals
  • a)
    1
  • b)
    √2
  • c)
    √3
  • d)
    2
Correct answer is option 'C'. Can you explain this answer?

Samarth Saha answered
The line has +ve and equal direction cosines, these are   or direction ratios are 1, 1, 1. Also thelines passes through P (2, – 1, 2).
∴ Equation  of line is
 be a point on this line where it meets the plane 2 x + y + z = 9
Then Q must satisfy the eqn of plane
∴ Q has coordintes (3, 0, 3)

Let  If the vectors   in the plane of  then x equals
  • a)
    – 4
  • b)
    – 2
  • c)
    0
  • d)
    1.
Correct answer is option 'B'. Can you explain this answer?

Anand Kumar answered
Since, vector a,b and c are coplanar.
Therefore, by condition of coplanarity,
vector c=m(vector a)+n(vector b)
substitute the given values of the individual vector.
further, relate coefficient of i,j and k of LHS to RHS
there is will have three relations between X,m and n . solve further for m,n and X.

Distance between two parallel planes 2x + y + 2z = 8 and 4x + 2y +4z + 5 = 0 is
  • a)
    9/2
  • b)
    5/2
  • c)
    7/2
  • d)
    3/2
Correct answer is option 'C'. Can you explain this answer?

Kiran Sengupta answered
To find the distance between two parallel planes, we need to find the perpendicular distance between them. The normal vectors of the planes will be parallel, so we can find the distance between the planes by finding the distance between a point on one plane and the other plane.

Given planes:
1) 2x + y + 2z = 8
2) 4x + 2y + 4z + 5 = 0

Step 1: Find the normal vectors of the planes
The normal vector of a plane is the coefficients of x, y, and z in the equation of the plane. So, the normal vectors for the given planes are:
1) Plane 1: (2, 1, 2)
2) Plane 2: (4, 2, 4)

Step 2: Find a point on one of the planes
To find a point on Plane 1, we can assume a value for one of the variables and solve for the other two variables. Let's assume x = 0. Substituting x = 0 in Plane 1 equation, we get:
y + 2z = 8
Assuming y = 0, we get:
2z = 8
z = 4
So, a point on Plane 1 is (0, 0, 4).

Step 3: Find the distance between the point and Plane 2
To find the distance between a point and a plane, we can use the formula:
Distance = |ax + by + cz + d| / √(a^2 + b^2 + c^2)
where (a, b, c) are the coefficients of x, y, and z in the equation of the plane, and d is the constant term.

Substituting the values for Plane 2, we get:
Distance = |4(0) + 2(0) + 4(4) + 5| / √(4^2 + 2^2 + 4^2)
= |16 + 5| / √(16 + 4 + 16)
= |21| / √(36)
= 21 / 6
= 7/2

Therefore, the distance between the two parallel planes is 7/2, which corresponds to option C.

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