# RD Sharma Class 12 Ex 29.15 Solutions Chapter 29 The Plane

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## RD Sharma Class 12 Ex 29.15 Solutions Chapter 29 The Plane

### Question 1. Find the image of the point (0, 0, 0) in the plane 3x + 4y – 6z + 1 = 0.

Solution:

According to the question we have

Plane = 3x + 4y – 6z + 1 = 0

Line passing through origin and perpendicular to plane is given by

So, let the image of (0, 0, 0) = (3r, 4r, -6r)

The midpoint of (0, 0, 0) and (3r, 4r, -6r) lies on the given plane

3(3r/2) + 2(4r) – 3(-6y) + 1 = 0

30.5y = -1

r = -2/61

So, the image is (-6/61, -8/61, 12/61)

### Question 2. Find the reflection of the point (1, 2, -1) in the plane 3x – 5y + 4z = 5

Solution:

According to the question we have to find the reflection of

the point P(1, 2, -1) in the plane 3x – 5y + 4z = 5

So, let Q = reflection of the point P

R = midpoint of PQ.

Then, R lies on the plane 3x – 5y + 4z = 5.

Now, the direction ratios of PQ are proportional to 3, -5, 4 and

PQ is passing through (1, 2, -1).

So, equation of PQ is,

Let Q be (3λ + 1, -5λ + 2, 4λ – 1)

The coordinates of R are =

Since, R lies on the given plane i.e., 3x – 5y + 4z = 5

Therefore,

9λ + 6 + 25λ – 20 + 16λ – 8 = 10

50λ – 22 = 10

50λ = 32

λ = 16/25

Q = (3λ + 1, -5λ + 2, 4λ -1)         -Equation(1)

Now, put the value of λ in equation (1), we get,

= (3(16/25)+1, -5(16/25)+2, 4(16/25)-1)

= ((48/25)+1,  (-16/5)+2, (64/25)-1)

= (73/25, -6/5, 39/25)

Hence, the reflection of point (1, 2, -1) = (73/25, -6/5, 39/25)

### Question 3. Find the coordinates of the foot of the perpendicular drawn from the point (5, 4, 2) to the line . Hence or otherwise deduce the length of the perpendicular.

Solution:

According to the question we have to find foot of the perpendicular, say Q,

drawn from point P(5, 4, 2) to the line

So, Let us assume Q = (2λ – 1, 3λ + 3, -λ + 1)          -Equation(1)

Direction ratio of line PQ are = (2λ – 6, 3λ – 1, -λ – 1)

Here, the line PQ is perpendicular to line the given line AB

So,

a1a2 + b1b2 + c1c2 = 0

(2λ – 6)(2) + (3λ – 1)(3) + (-λ – 1)(-1) = 0

4λ – 12 + 9λ – 3 + λ + 1 = 0

14λ – 14 = 0

λ = 14/14

λ = 1

So, put the value of λ in equation(1), we get

= (2(1) – 1, 3(1) + 3, -(1) + 1)

= (2 – 1, 3 + 3, -1 +1)

= (1, 6, 0)

Now, we find the length of perpendicular PQ using distance formula

= √24

= 2√6

So, the foot of the perpendicular is (1, 6, 0)

Length of the perpendicular is 2√6 units.

### Question 4. Find the image of the point with position vector  in the plane  . Also find the position vector of the foot of the perpendicular and the equation of the perpendicular line through  .

Solution:

According to the question we have to find image of the point P(3, 1, 2)

in the plane  or 2x – y + z = 4.

Let Q be the image of the point P.

So,

The direction ratios of normal to the point = 2, -1, 1

The direction ratios of line PQ perpendicular to 2, -1, 1 and

PQ is passing through (3, 1, 2)

So equation of PQ is

General point on the line PQ is = (2λ + 3, -λ + 1, λ + 2)

Let us assume Q = (2λ + 3, -λ + 1, λ + 2)           -Equation(1)

Let R be the mid point of PQ. Then,

Coordinates of R =

Since, R lies on the plane 2x – y + z = 4, we get

4λ + 12 + λ – 2 + λ + 4 = 8

6λ = 8 – 14

λ = -6/6

λ = -1

So, put the value of λ in equation(1), we get

Image of P = Q(2 (-1) + 3, – (-1) + 1, -1 + 2)

Image of P = (1, 2, 1)

Equation of the perpendicular line through  is

Position vector of the image point is

Position vector of the foot of the perpendicular is

By putting the value of λ in the position vector of the foot of the perpendicular is

### Question 5. Find the coordinates of the foot of the perpendicular from the point (1, 1, 2) to the plane 2x – 2y + 4z + 5 = 0. Also, find the length of the perpendicular.

Solution:

According to the question we have,

Plane = 2x – 2y + 4z + 5 = 0         -Equation(1)

Point = (1, 1, 2)

and find the coordinates of the foot of the perpendicular

Let us assume that the foot of perpendicular = (x, y, z).

So, DR’s are in proportional

x = 2k + 1

y = -2k + 1

z = 4k + 2

Substitute (x, y, z) = (2k + 1, -2k + 1, 4k + 2) in the equation(1), we get

2x – 2y + 4z + 5 = 0

4k + 2 + 4k – 2 + 16k + 8 + 5 = 0

24k = -13

k = -13/24

So, the coordinates of the foot of the perpendicular (x, y, z) = (-1/12, 5/3, -1/6)

### Question 6. Find the distance of the point (1, -2, 3) from the plane x – y + z + 5 measured along a line parallel to

Solution:

According to the question, we have to find the distance of point P(1, -2, 3)

from the plane x – y + z = 5 measured

parallel to line AB,

Let us assume Q = Mid point of the line joining P to plane.

We have, PQ parallel to line AB

The direction ratios of line PQ are proportional to direction ratios of line AB

The direction ratios of line PQ = 2, 3, -6

PQ is passing through point P(1, -2, 3).

Thus, the equation of PQ is,

The general point on the line PQ = (2λ + 1, 3λ – 2, -6λ + 3)

Suppose the coordinates of Q = (2λ + 1, 3λ – 2, -6λ + 3)

Thus, Q lies on the plane x – y + z = 5

(2λ + 1) – (3λ – 2) + (-6λ + 3) = 5

2λ + 1 – 3λ + 2 – 6λ + 3 = 5

-7λ = -1

λ = 1/7

Coordinate of Q = (2λ + 1, 3λ – 2,  -6λ + 3)          -Equation(1)

Now, put the value of λ in equation(1), we get

Q = (2(1/7)+1, 3(1/7)-2, -6(1/7)+3)

Q = (9/7, -11/7, 15/7)

Now, we find the distance between (1, -2, 3) and plane = PQ

= 1

Hence, the required distance is 1 unit.

### Question 7. Find the coordinates of the foot the perpendicular from the point (2, 3, 7) to the plane 3x – y – z = 7. Also, find the length of the perpendicular.

Solution:

Let us assume that Q be the foot of the perpendicular.

Now, the direction ratios of normal plane is 3, -1, -1

Line PQ is parallel to normal to plane

Direction ratios of PQ are proportional to 3, -1, -1

PQ is passing through point P(2, 3, 7)

So,

The general point on the line PQ

= (3λ + 2, -λ + 3, -λ + 7)

Coordinates of Q = (3λ + 2, -λ + 3, -λ + 7)         -Equation(1)

Point Q lies on the plane 3x – y – z = 7

Thus,

3(3λ + 2) – (-λ + 3) – (-λ + 7) = 7

9λ + 6 + λ – 3 + λ – 7 = 7

11λ = 7 + 4

11λ = 11

λ = 11/11

λ = 1

Now, put the value of λ in equation(1), we get

Q = (3(1) + 2, -(1) + 3, -(1) + 7)

Q = (5, 2, 6)

Find the length of the perpendicular PQ

= √11

### Question 8. Find the image of the point (1, 3, 4) in the plane 2x – y + z + 3 = 0.

Solution:

According to the question we have to find the image of point P(1, 3, 4)

in the plane 2x – y + z +3 = 0

Now let us assume that Q be the image of the point.

Here, the direction ratios of normal to plane = 2, -1, 1

The direction ratios of PQ which is parallel to the normal to the plane

is proportional to 2, -1, 1 and the line PQ is passing through point P(1, 3, 4).

Thus, equation of the line PQ is:

Now, the general point on the line PQ = (2λ + 1, -λ + 3, λ + 4)

Let Q = (2λ + 1, -λ + 3, λ + 4)          -Equation(1)

Here, Q is the image of P, so R is the mid point of PQ

Coordinates of R =

Point R is lies on the plane 2x – y + z + 3 = 0

= 2(λ + 1) –

4λ + 4 + λ – 6 + λ + 8 + 6 = 0

6λ = -12

λ = -2

Now, put the value of λ in equation(1), we get

= (-4 + 1, 2 + 3, -2 + 4)

= (-3, 5, 2)

Hence, the image of point P(1, 3, 4) is (-3, 5, 2)

### Question 9. Find the distance of the point with position vector from the point of intersection of the line with the plane .

Solution:

According to the question we have to find distance of a point A with position

vector from the point of intersection of

line

with plane

Let the point of intersection of line and plan be

The line and the plane will intersect when,

(2 + 3λ)(1) + (-1 + 4λ)(-1) + (2 + 12λ)(1) = 5

2 + 3λ + 1 – 4λ + 2 + 12λ = 5

11λ = 5 – 5

λ = 0

So, the point B is given by

The required distance is 13 units.

### Question 10. Find the length and the foot of the perpendicular from the point (1, 1, 2) to the plane .

Solution:

Plane = x – 2y + 4z + 5 = 0          -Equation(1)

Point = (1, 1, 2)

D =

= 12/√21

The length of the perpendicular from the given point to the plane = 12/√21

Let us assume that the foot of perpendicular be (x, y, z).

So DR’s are in proportional

x = k + 1

y = -2k + 1

z = 4k + 2

Substitute (x, y, z) = (k + 1, -2k + 1, 4k + 2) in the plane equation(1)

k + 1 + 4k – 2 + 16k + 8 + 5 = 0

21k = -12

k = -12/21 = -4/7

Hence, the coordinate of the foot of the perpendicular  = (3/7, 15/7, -2/7)

### Question 11. Find the coordinates of the foot of the perpendicular and the perpendicular distance of the point P(3, 2, 1) from the plane 2x – y + z + 1 = 0. Find also the image of the point in the plane.

Solution:

Given:

Plane = 2x – y + z + 1 = 0          -Equation(1)

Point P = (3, 2, 1)

D =

The perpendicular distance of the point P from the plane(D) = √6

Let us assume that the foot of perpendicular be (x, y, z).

So DR’s are in proportional

x = 2k + 3

y = -k + 2

z = k + 1

Substitute (x, y, z) = (2k + 3, -k + 2, k + 1) in the plane equation(1)

4k + 6 + k – 2 + k + 1 + 1 = 0

6k = -6

k = -6/6 = -1

The coordinate of the foot of the perpendicular = (1, 3, 0)

### Question 12. Find the direction cosines of the unit vector perpendicular to the plane passing through the origin.

Solution:

Given:

Equation of the plane

Thus, the direction ratios normal to the plane are 6, -3 and -2

Hence, the direction cosines to the normal to the plane are

= 6/7, -3/7, -2/7

= -6/7, 3/7, 2/7

The direction cosines of the unit vector perpendicular to the plane

are same as the direction cosines of the unit vector perpendicular

to the plane are: -6/7, 3/7, 2/7

### Question 13. Find the coordinates of the foot of the perpendicular drawn from the origin to the plane 2x – 3y + 4z – 6 = 0.

Solution:

According to the question,

Plane = 2x – 3y + 4z – 6 = 0

The direction ratios of the normal to the plane are 2, -3 and 4.

Thus, the direction ratios of the line perpendicular to the plane are 2, -3 and 4.

The equation of the line passing (x1, y1, z1) having direction ratios a, b and c is

Thus, the equation of the line passing through the origin

with direction ratios 2, -3 and 4 is

Here, r is same constant.

Any point on the line is of the form 2r, -3r, and 4r,

if the point P(2r, -3r, 4r) lies on the plane 2x – 3y + 4z – 6 = 0.

Thus, we have,

2(2r) – 3(-3r) + 4(4r) – 6 = 0

4r + 9r + 16r – 6 = 0

29r = 6

r = 6/29

Thus, the coordinates of the point of intersection of the perpendicular

from the origin and the plane are:

P(2×6/29, -3×629, 4×6/29) = P(12/29, -18/29, 24/29)

### Question 14. Find the length and the foot of the perpendicular from the point (1, 3/2, 2) to the plane 2x – 2y + 4z +5 = 0.

Solution:

Given:

Point = (1, 3/2, 2)

Plane = 2x – 2y + 4z + 5 = 0

D =

= √6

So, the length of the perpendicular from the point to the plane(D) = √6

Let the foot of perpendicular be (x, y, z). So, DR’s are in proportional

x = 2k + 1

y = -2k + 3/2

z = 4k + 2

So, using the values of x, y, z in equation of the plane we have,

2(2k + 1) – 2(-2k + 2/3) +4(4k + 2) + 5 = 0

4k + 2 + 4k – 3 + 16k + 8 + 5 = 0

24k = -12

k = -1/2

So, the coordinate of the foot of the perpendicular = (0, 5/2, 0)

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