All questions of Dimensional Geometry for Mathematics Exam

xx1 + yy1 = a2 is the equation of the
  • a)
    Tangent to the circle x2+ y2=a2 at point (x1,y1)
  • b)
    chord of contact of tangents drawn from the external point (x1, y1) to the circle x2+ y2 = a2
  • c)
    polar of the point (x1, y1) w.r.t the circle x2 + y2 = a2
  • d)
    all of the above
Correct answer is option 'D'. Can you explain this answer?

Vivek Kumar answered
Remark : 1. The following three equations coincide:
(i) Equation of the tangent to a circle at a point P(x1, y1)
(ii) Equation of the chord of contact of the tangents to the circle from an outside point (x1, y1)
(iii) Equation of the polar of a point (x,  y) inside or outside the circle (but not on the circle), with repsect to the circle.
Remark: 2. If the point P(x1, y1) is on the circle, then its polar coincides with the tangent at that point.
Remark: 3. If the polar of a point P passes through a point Q, then the polar of Q passes through P.

If two pairs of opposite edges of a tetrahedron art; at right angles, then the third pair
  • a)
    is also at right angles always
  • b)
    can never be at right angle
  • c)
    is inclined at. some acute angle
  • d)
    is inclined at some obtuse an
Correct answer is option 'A'. Can you explain this answer?

Chirag Verma answered
Let the coordinates of the vertices O, A, B, C o f tetrahedron be (α, β, γ), (a, 0, 0), (0, b, 0) and (0. 0, c) respectively. Then it is given that
OA is perpendicular to BC
and OB is perpendicular to CA
To prove that
OC is perpendicular to AB

What is the locus of a point for which x = a and y= b?
  • a)
    a line parallel to XY plane
  • b)
    a plane parallel to XY plane
  • c)
    a line parallel to z-axis
  • d)
    a plane perpendicular tpo XY plane
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
x = a is a plane | | to YZ plane
y = b is a plane | | to ZX plane
Therefore x = a and y = b is a line parallel to z-axis
Note : Remember that two planes intersect in a line

The distance between the parallel planes 2x -2y + z + 3 = 0 and 4x -4y + 2z + 5 = 0 is
  • a)
    1/2
  • b)
    1/3
  • c)
    1/6
  • d)
    none of the above
Correct answer is option 'C'. Can you explain this answer?

Veda Institute answered
To find the distance between two parallel planes, we follow the method given below. Method:Find the perpendicular distance of each plane from the origin with proper sign (i.e. do not. lake the mod). Ther. their differeee is the required distance between two parallel planes. Let d. and d, be the distances of the planes 
2x -2y + z + 3 =0 ...(i)
and 4x - 4y + 2z + 5 = 0 ...(ii)
from the origin. Then

The equation
ax2 + ay2 + az2 + 2ux - 2vy - 2wz + d - 0 (a ≠ 0)
represents a sphere if
  • a)
    u2+v2+w2-d≥0
  • b)
    u2+v2+w2-ad≥0
  • c)
    u2+v2+w2-d≤0
  • d)
    u2+v2+w2-ad≤0
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
The given equation is

Note that the equation of a sphere has three characteristics :
1. it is of second degree in x, y, y
2. the coefficients of x2, y2 and z2 and equal
3. the product icrins xy, yzand zx are absent. 

The equation of the plane passing through the point (x1, y1, z1) and through the line 
  • a)
  • b)
  • c)
  • d)
    none of the above
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
To find the equation of a plane passing through a given point (x1, y1, z1) and a given line 
The equation of the plane through the line (i) is 

Since the plane (ii) passes through the point (x1, y1, z1), therefore

Eliminating a, b, c from equations (ii), (iii) and (iv), we get the required equation of the plane as

What is the magnitude of the line of shortest distance between the lines  
  • a)
    √14
  • b)
    14
  • c)
    7
  • d)
    2
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
Magnitude and equations of Shortest Distance (S.D)
Here we have to find the magnitude and the equation of shorttest distance between two given lines.

*    It
By definition, the line of shortest distance between (i) and (ii) is perpendicular to both (i) and (ii). Therefore, if [λ,μ,ν] an; the d.c.’s of the line of shortest distance we have


Remark: Two lines given by (i) and (ii) will intersect, i.e. these lines will be coplanar if shortest distance between them is zero. Thus the condition for two lines to be coplanar is
 

To transform the equations of a line from unsymmeirieal form to the symmetrical form, it is necessary to know about
  • a)
    The direction ratio of the line
  • b)
    The coordinates of any one point on it
  • c)
    The coordinates of any two points on it and its direction ratio
  • d)
    Its direction ratios and the coordinates of any one point on it only
Correct answer is option 'D'. Can you explain this answer?

Chirag Verma answered
The equations of the line in the symmetrical form are

where (x1, y1 ,z1) is a given point on the line and (l, m, n) are the direction ratios of the line. Therefore the equations of a straight line can be written in symmetrical form if we know.
(i) the direction ratios of the line and
(ii) a point on it.

The angle between the line  and the plane ax+by+cz+d=0, is given by
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
To find the angle between the line


Let θ be 1 he angle between the line (i) and the plane (ii). Then 90 - θ is the angle between the line and the normal to the plane.
Now th2 d.r.’s of the line (i) are l, m, n and the d.r/s of the line perpendicular lo (ii) are a, b, c
∴ cos (90 - θ)

Which is a wrong statement? The three planes
a1x + b1y + c1z + d1 = 0
a2x + b2y + c2z + d2 = 0
a3x + b3y + c3z + d3 = 0
have a common line of intersection, if
  • a)
  • b)
  • c)
     all vanish
  • d)
Correct answer is option 'A'. Can you explain this answer?

Chirag Verma answered
 The three planes are

Denote the determinants as follows:
   
Note that Δ is th e coefficients of x, y and z . Δ1, Δ2 and Δ3, are obtained from Δ by replacing its first column, 2nd column and 3rd column by d1, d2 and d3, respectively.
Also note that if the three planes are to intersect in a line, then no two of them are parallel. Under this case, the line of intersection of any two planes will lie on the third plane. Now the line of intersection of (i) and (ii) is given by

Since line (iv) lies on plane (iii), therefore is perpendicuar to the normal to the plane (iii)


Also since line (iv) lies on plane (iii), therefore the coordinates of any point on the line will satisfy the equation of plane (iii), since



Thus (A) and (R) are the required conditions for the three planes to have a common line of intersection.
In a similar way, it can he shown that Δ = 0 and Δ1 = 0 or Δ = 0 and Δ2 = 0 are also the required conditions.
∴ (b), (c), (d) are correct.
Remark: To prove that three planes intersect in a line, verify that Δ = 0 and any one: of Δ12 and Δ
3 
becomes zero.

The equation of a straight line which makes an angle of 60° with x-axis and passes through the point (√3, 2) is given by
  • a)
    y = √3x + 1
  • b)
    √3x- y = 1
  • c)
    x - √y = √3
  • d)
    x-√2y=√2
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
The required equation
(i) makes an angle of 60° with x-axis
(ii) passes through the point (√3 , 2)
Now
(i) ⇒ equation of the straight line will be
y = tan 60° x + c
or y = √3 x + c ...(i)
(ii) ⇒ the coordinates of the point will satisfy equation (i)

Let A(-1,2, -3), B(5, 0, -6), C(0, 4, -1) be the three points. Then the direction cosines of the internal bisector of the angle BAC are proportional to
  • a)
    (25,8,5)
  • b)
    (-11,20,23)
  • c)
    (7,-6,-9)
  • d)
    none of the above
Correct answer is option 'A'. Can you explain this answer?

Chirag Verma answered
Proof in short : verify that
AB = 7 and AC = 3


where θ is the angle between AB (with d.c.'s [l1, m1, n1]) and AC (with d.c.’s [l2, m2, n2)
∴ the d.c.’s of the internal bisector are proprotional to l1+l2, m1+m2, n1+n2
or proportional to 
or proportional to 
or proportional to 25, 8, 5

The two planes represented by ax2 - by2 + cz2 + 2fyz + 2gzx + 2 hxy = 0 are perpendicular if
  • a)
    abc + 2fgh + af2 + bg- + ch2 = 0
  • b)
    af + bg + ch - 0
  • c)
    a + b + c = 0
  • d)
    f2 + g2 + h2 - bc - ca - ab = 0
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
Let the two planes r epresented by




Condition of perpendicularity.
The two planes are perpendicular if
θ = 90° or tan θ = ∞
or a + b + c = 0
Remark: since the two planes pass through the origin, therefore the two planes are never parallel. The two planes can however coincide.

The distance of the plane 6x -3y + 2z - 14 = 0 from the origin is
  • a)
    2
  • b)
    14
  • c)
    -14
  • d)
    7
Correct answer is option 'A'. Can you explain this answer?

Chirag Verma answered
The length of the perpendicular from (x1, y1 ,z1) to the plane Ax+ By - Cz + D = 0  
is given by

∴ perpendicular distance d of the plane
6x-3y-2z -14=0
from the origin is given by

The three planes 
2x + 3y - z - 2 = 0
3x + 3y + z - 4 - 0
x - y + 2z - 5 = 0
intersect in
  • a)
    A prism
  • b)
    A point
  • c)
    A line
  • d)
    none of the above
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
The equations of the planes are

Let us calculate the determinant A of coefficients.

Since A ≠ 0, therefore the planes will intersect at a point.

The equation of the bisector of the angle between the lines 3x - 4y + 7 = 0 and 12x - 3y -8 =0 , in w hich the origin lines, is given by
  • a)
    21x+27y-131=0
  • b)
    99x-77y+51=0
  • c)
    21x+27y+131=0
  • d)
    99x-77y-51=0
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
Equations of Bisectors of the Angles between two non-parallel lines.
Let the straight lines
Ax+ By + C = 0 and
A'x + B'y + C = 0
be non-parallel. Then the bisectors are

Rules:
1. if C > 0, C' > 0 and AA' + BB' > 0, then (i) is the equation of obtuse - angle bisector (so the equation (ii) gives the acute - angle bisector).
2. If C > 0, C' > 0 and AA' * BB' < 0, then (i) is the equation of acute - angle bisector (so (ii) gives the obtuse - angle bisector)
3. If C and C' are of the same sign (cither both positive or both negative), then (i) is the bisector of that angle (acute or obtuse) in which the origin lies.
In Problem 42, the given equations are
3x-4y + 7= 0 and
12x - 5y = 8 = 0
Since C and C' are of opposite sign, therefore the bisector of the angle between (iii) and (iv), in which the origin lies in given by

Which of the following equation represent pair of perpendicular planes?
  • a)
    2x2 - 2y2 + 4z2 - 2yz+ 6xz - 3xy = 0
  • b)
    6x2+4y2-10z2+3yz+4zx-11xy=0
  • c)
    12x2-y2+6z2+7yz+6zx+2xy=0
  • d)
    12x2-y2+2z2-yz+5zx+xy=0
Correct answer is option 'B'. Can you explain this answer?

Veda Institute answered
Planes represented by (b) are perpendicular planes.
Proof: The planes are given by
6x2+4y2-10z2+3yz+4zx-11xy=0
∴ a=6, b=4, c=-10
∴ a+b+c=6-4-10=0
Remark. Two planes arc perpendicular if the sum of the coefficients of x2, y2 and z2 is zero.

The radical axis of the two circles
x2 + y2 +2gx + 2fy + c = 0 and
x2 + y2 + 2g1x + 2f1y + c1 =0 is given by
  • a)
    x(g-g1) + y(f-f1) +( c-c1) =0
  • b)
    x(g-g1) + y(f-f1) +2( c-c1) =0
  • c)
    x(g-g1) + y(f+f1) +( c-c1) =0
  • d)
    2x(g-g1) + 2y(f-f1) +( c-c1) =0
Correct answer is option 'D'. Can you explain this answer?

Veda Institute answered
Comments about Radical Axis
1. Definition : The radical axis of two circles is the locus of a point which moves in such a way that the lengths of the tangents drawn from it to the two circles are equal.
2. The radical axis of two circles is perpendicular !<> the line joining their centres.
3. Equation of the Radical Axis
Let [h, k) be the point on the radical axis. Then the lengths of the tangents from (h, k) to the two circles are equal.

If the projection of a line segment on the X- axis , Y-axis and Z axis be 12, 4 and 3 respectiveIv, then it’s length will be
  • a)
    19
  • b)
    13
  • c)
    17
  • d)
    none
Correct answer is option 'B'. Can you explain this answer?

Pranav Sharma answered
Is not possible to determine the length of the line segment solely based on the information given. The projection lengths on the X-axis, Y-axis, and Z-axis only provide information about the components of the line segment along each axis, but they do not give the actual length of the line segment.

The equation of the plane through the points (2, 3, 1) and (-4. 5. 3) and parallel to X - axis is given by
  • a)
    x - z - 1 = 0
  • b)
    4x - y - 11 = 0
  • c)
    y + 4z - 7 = 0
  • d)
    6x - 2y - 2x + 4 = 0
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
The plane is parallel' to x-axis. Therefore, it is perpendicular to yzplane.
Hence its equation should be of the form 
By Cz + D = 0
Since, the plane passes through (2, 3, 1) and (4, -5, 3), therefore.

The two planes ax + by +by + cz + d = 0 and a'x + b'y + c'z + d = 0 are perpendicular if
  • a)
    aa' + bb' + cc' +dd' =0
  • b)
    a/a' = b/b' + c/c' + d/d' =0 
  • c)
    a/a'=b/b' =c/c'
  • d)
    aa' +bb' + cc' =0
Correct answer is option 'D'. Can you explain this answer?

Rajat Agrawal answered
Explanation:

To determine if two planes are perpendicular, we need to check the dot product of their normal vectors. Let's consider the two planes:

Plane 1: ax + by + cz + d = 0
Plane 2: ax + by + cz + d = 0

Step 1: Finding the normal vectors:
The coefficients of x, y, and z in the equations of the planes give us the normal vectors. In this case, the normal vectors for both planes are given by:

Normal vector 1: (a, b, c)
Normal vector 2: (a, b, c)

Step 2: Calculating the dot product:
The dot product of two vectors is given by the sum of the products of their corresponding components. In this case, the dot product of the normal vectors is:

(a, b, c) • (a, b, c) = a*a + b*b + c*c

Step 3: Checking for perpendicularity:
For two vectors to be perpendicular, their dot product must be zero. Therefore, we need to check if:

a*a + b*b + c*c = 0

Step 4: Simplifying the equation:
To simplify the equation further, we can divide both sides by a*a, b*b, and c*c (assuming they are not zero):

(a*a + b*b + c*c)/(a*a) = 0/(a*a)
1 + (b*b)/(a*a) + (c*c)/(a*a) = 0

Step 5: Simplifying further:
We can rewrite (b*b)/(a*a) as (b/a)^2 and (c*c)/(a*a) as (c/a)^2:

1 + (b/a)^2 + (c/a)^2 = 0

Step 6: Final conclusion:
For the equation to hold true, each of the terms on the left side must be zero. Therefore, we have:

a = 0
b/a = 0
c/a = 0

This implies that a, b, and c are all zero. Hence, the correct answer is option 'D': aa + bb + cc = 0.

If each of the pairs of straight lines x2 - 2pxy - y2 = 0 and x2 - 2qxy- y2 = 0 bisects the angles between the other pair, then
  • a)
    pq=1
  • b)
    pq=-1
  • c)
    p2=-q2
  • d)
    p2q2 = -1
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
The equation of the bisectors of the angles between the two straight lines given by ax2+2hxy+by2=0 is given by

Now we are given two pairs of straight lines, namely,
x2- 2 pxy - y2 = 0 ...(I)
and x2- 2qxy-y2 = 0 ...(II)
It is given that each of I or II is the bisectors of the angle between the other pair. Using (A) the equation of the bisectors of the angle between the straight lines represented by (I) is given by


The equation of the plane containing the lines   is not given by
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Chirag Verma answered
To find the equation of the plane containing the lines

The plant: containing (i) is
A ( x - a) + B ( y - b) + C ( z - c) = 0 ...(iii)
where Aa' + Bb‘ + Cc' 3 =0 ...(iv)
Further, since the plane (iii) also contains line (ii),  therefore , the normal to plane (iii) is perpendicular to line (ii).
∴ Aa + Bb + Cc = 0 ...(v)
Eliminating A, B, C from equations (iii), (iv) and (v), we get the required equation of plane 

This is statement (b). The statements (a) and (c) can be deduced from this by simple propert ies of determinants. Thus (a), (b|, (c) are correct.

The point of intersection of the lines drawn from the vertices of any tetrahedron to the centroids of the opposite faces divides the distance from each vertex to the opposite face in which of the following ratios?
  • a)
    3:1
  • b)
    3:4
  • c)
    2:1
  • d)
    2:3
Correct answer is option 'A'. Can you explain this answer?

Chirag Verma answered
About Direction Cosines and Direction Ratios.
1. If a given line makes angles α, β, γ with positive directions of the axes of x, y and x respectively, then cosα, cosβ, cosγ are called the direction cosines (in short d.c.’s) of the given line.
Direction cosines of a line are generally denoted by l, m, n and are written as [l, m, n]

where r is the distance between the given points.
In particular, the d.c.'s of a line joining a point (a1, y1, z1) to the origin are

4. Direction Cosines satisfy


Thus if the d.r.’s of a line are a,b, c, then the direction cosines of the line are

The equation of the circle, passing through the origin such that the x-axis is its diameter, is given by
  • a)
    x2 + y2 - 2ky = 0
  • b)
    x2 + y2 - 2hx = 0
  • c)
    x2 + y2 - 2hx + h2 = 0
  • d)
    x2 + y2 - 2ky + k2 = 0
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
Since the circle passes through the origin and has x-axis as its diameter, therefore its centre is [h, 0) and radius = h
∴ Required equation of the circle is
(x-h)2 + (y-0)2 = h2
or x2 + y2 -2hx = 0

The equation of the straight line passing through (4, 5) and parallel to the line 2x - 3y = 5 is given by
  • a)
    2 x - 3 y = 7
  • b)
    2 x - 3 y = -7
  • c)
    3 x - 2 y = 7
  • d)
    3 x - 2 y = -7
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
 The given straight line is 

The equation of a straight line parallel to (i) will be

Since (ii) passes through P(4, 5), therefore the coordinates of P A\*ill satisfy equation (ii)

∴ Required equation of the straight line is

What is the locus of a point for which y=b?
  • a)
    A plane parallel to ZX plane at a distance b from it
  • b)
    A plane parallel to YX plane at a distance b from it
  • c)
    A plane parallel to YZ plane at a distance b from it
  • d)
    A line parallel to X axis at a distance b from it
Correct answer is option 'A'. Can you explain this answer?

Veda Institute answered
Note: We have
  1. Plane | | to xy piane at a distance c, z = c
  2. Plane; | | to t/x plane ai a distance a, x = a
  3. Plane | to xx plane at a distance b ,y = b
Remark:- In particular, the equations of YZ, ZX and XY planes are x = 0, y = 0 and z = 0 respectively.

The polar equation of a line through two given points (r1, θ1) and (r2, θ2) is given by
  • a)
    r1r2[sin(θ21] + sin(θ-θ2) + sin(θ1-θ)] =0
  • b)
    r1r2[cos(θ21] + cos(θ-θ2) + cos(θ1-θ)] =0
  • c)
    r1r2[sin(θ12] +r2 rsin(θ2-θ) + rr1sin(θ-θ1)] =0
  • d)
    none of the above
Correct answer is option 'C'. Can you explain this answer?

Chirag Verma answered
A (r1, θ) and B(r2, θ2) arc two given points through them passes the straight line L. Let P(r, θ) be an arbitrary point on L.

Then the points P, A and B are collinear.
=>the area of the triangle formed by these three points is zero

The equation of the plane passing through (2, -3, 1) and perpendicular to the line joining the points (3, 4. -1) and (2, -1, 5) is given by
  • a)
    5x+ 3y + 4z -5 = 0
  • b)
    x + 5y - 6z + 19 = 0
  • c)
    5x + 3y + 4z + 5 = 0
  • d)
    x + 5y +6z - 19 = 0
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
Let P(x, y, z) be an arbitrary point on the plane. Then the line PA joining the points P(x, y, z) and A(2, -3, 1) lies on the plane and the d.r.’s of PA are
x - 2, y + 3, z- 1
Further, the direction ratios of the line joining points B(3, 4. - 1 ) and C(2, 1,5) are
3 - 2 , 4 - ( - 1 ),- 1 -5 
i.e. 1, 5, -6
Since BC is perpendicular to the plane and hence is perpendicular to line PA, therefore
( x - 2) 1 + (y + 3) 5 + ( z - 1) (- 6 ) = 0
or x + 5y - 6z + 19 = 0

The area of the triangle ABC formed by the intercepts OA a, OB = h, OC = con the coordinate axes respectively by a plane is given by
  • a)
  • b)
  • c)
  • d)
    none of the above
Correct answer is option 'C'. Can you explain this answer?

Veda Institute answered
If Ax, Ay, Az be the areas of projection of an area A on the three coordinate planes then A2=Ax2 + Ay2 + Az
we have to find the area A of triangle ABC Ax =  the projection of A on yz plane 
Ay = the projection of A on zx plane 
Az = the projection of A on xy plane 
∴ A2=Ax2 + Ay2 + Az2

Projection on line segment joining P(6, 3, 2) and Q(5, 1, 4) on the line AB, where A and B are (3, -4, 7) and (0, 2, 5) respectively is
  • a)
    13
  • b)
    -13/7
  • c)
    7
  • d)
    none
Correct answer is option 'B'. Can you explain this answer?

Chirag Verma answered
Direction cosines of the line joining A{3, -4, 7) and B(0, 2, 5) are given by


Now the required projection of line joining P(6, 3, 2) and Q(5, 1. 4) on the line AB is given by

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