# Class 12 Maths NCERT Solutions for Chapter 11 Three Dimensional Geometry Miscellaneous Exercise

### Three Dimensional Geometry Miscellaneous Exercise Solutions

**1. Show that the line joining the origin to the point (2, 1, 1) is perpendicular to the line determined by the points (3, 5, – 1), (4, 3, – 1).**

**Solution**

Let OA be the line joining the origin, O (0, 0, 0), and the point, A (2, 1, 1).

Also, let BC be the line joining the points, B (3, 5, −1) and C (4, 3, −1).

The direction ratios of OA are 2, 1, and 1 and of BC are (4 − 3) = 1, (3 − 5) = −2, and (−1 + 1) = 0

OA is perpendicular to BC, if *a*_{1}*a*_{2} + *b*_{1}*b*_{2} + *c*_{1}*c*_{2} = 0

∴ *a*_{1}*a*_{2} + *b*_{1}*b*_{2} + *c*_{1}*c*_{2} = 2×1 + 1 (−2) + 1×0 = 2 − 2 = 0

Thus, OA is perpendicular to BC.

**2. If l_{1}, m_{1}, n_{1} and l_{2}, m_{2}, n_{2} are the direction cosines of two mutually perpendicular lines, show that the direction cosines of the line perpendicular to both of these are m_{1}n_{2} − m_{2}n_{1}, n_{1}l_{2} − n_{2}l_{1}, l_{1}m_{2} − l_{2}m_{1}. **

**Solution**

It is given that *l*_{1}, *m*_{1}, *n*_{1} and *l*_{2}, *m*_{2}, *n*_{2} are the direction cosines of two mutually perpendicular lines. Therefore,

Let l, m, n be the direction cosines of the line which is perpendicular to the line with direction cosines *l*_{1}, *m*_{1}, *n*_{1} and *l*_{2}, *m*_{2}, *n*_{2} .

∴ *l**l*_{1}, *m**m*_{1}, *n**n*_{1} = 0 *ll*_{2}, *m**m*_{2}, *n**n*_{2} = 0 *l, *m, n are the direction cosines of the line.

Therefore, l^{2} + m^{2} + n^{2} = 1 **…(5)**

It is known that,

**3. Find the angle between the lines whose direction ratios are a, b, c and b−c, c−a, a−b.**

**Solution**

The angle Q between the lines with direction cosines, a, b, c and b-c, c-a, a-b is given by,

Thus, the angle between the lines is 90°.

**4. Find the equation of a line parallel to x-axis and passing through the origin.**

**Solution**

The line parallel to *x*-axis and passing through the origin is *x*-axis itself.

Let A be a point on *x*-axis. Therefore, the coordinates of A are given by (*a*, 0, 0), where *a* ∈ R.

Direction ratios of OA are (*a* − 0) = *a*, 0, 0

The equation of OA is given by,

Thus, the equation of line parallel to x - axis and passing through origin is

x/1 = y/0 = z/0

**5. If the coordinates of the points A, B, C, D be (1, 2, 3), (4, 5, 7), (−4, 3, −6) and (2, 9, 2) respectively, then find the angle between the lines AB and CD.**

**Solution**

The coordinates of A, B, C, and D are (1, 2, 3), (4, 5, 7), (−4, 3, −6), and (2, 9, 2) respectively.

The direction ratios of AB are (4 − 1) = 3, (5 − 2) = 3, and (7 − 3) = 4

The direction ratios of CD are (2 −(− 4)) = 6, (9 − 3) = 6, and (2 −(−6)) = 8

It can be seen that,

Therefore, AB is parallel to CD.

Thus, the angle between AB and CD is either 0° or 180°.

**6. If the lines (x - 1)/-3 = (y - 2)/2k = (z - 3)/2 and (x -1)/3k = (y - 1)/1 = (z - 6)/-5 are perpendicular, find the value of k. **

**Solution**

The direction of ratios of the lines, (x - 1)/-3 = (y - 2)/2k = (z - 3)/2 and (x -1)/3k = (y - 1)/1 = (z - 6)/-5 are -3, 2k, 2 and 3k, 1, -5 respectively.

It is known that two lines with direction ratios, a_{1} , b_{1} , c_{1} ,and a_{2} , b_{2} , c_{2} are perpendicular , if a_{1}a_{2} + b_{1}b_{2} + c_{1}c_{2} = 0

∴ -3(3k) + 2k × 1 + 2(-5) = 0

⇒ -9k + 2k - 10 = 0

⇒ 7k = -10

⇒ k = -10/7

Therefore, for k = -10/7 , the given lines are perpendicular to each other.

**7. Find the vector equation of the plane passing through (1, 2, 3) and perpendicular to the plane r⃗ .(i ^+ 2j ^- 5k ^)+9 = 0**

**Solution**

The position vector of the point (1, 2, 3) is r1⃗ = i ^ + 2j ^ + 3k ^ .

The direction ratios of the normal to the plane, r⃗.(i ^ + 2j ^ - 5k ^) + 9 = 0 are 1, 2, and -5 and the normal vector is n⃗ = (i ^ + 2j ^ - 5k ^)

The equation of a line passing through a point and perpendicular to the given plane is given by,

**8. Find the equation of the plane passing through (a, b, c) and parallel to the plane r⃗. (i ^+ j ^+ k ^) =2 **

**Solution**

Any plane parallel to the plane, r1⃗ .(i ^+ j ^ + k ^) = 2 , is of the form

r⃗ .(i ^ + j ^ + k ^) = Î» **...(1) **

The plane passes through the point (a, b, c). Therefore, the position vector r⃗ of this point

r⃗ = ai ^ + bj ^ + ck ^

Therefore, equation (1) becomes

(ai ^ + bj ^ + ck ^).(i ^ + j ^ + k ^) = Î»

⇒ a + b + c = Î»

Substituting Î» = a + b + c in equation (1), we obtain

r⃗ .(i ^ + j ^ + k ^) = a + b + c **...(2) **

This is the vector equation of the required plane.

Substituting r⃗ = xi ^ + yj ^ + zk ^ in equation (2), we obtain

(xi ^ + yj ^ + zk ^).(i ^ + j ^ + k ^) = a + b + c

⇒ x + y + z = a + b+ c

**9. Find the shortest distance between lines r⃗ = 6i ^+ 2j ^+ 2k ^+ Î»(i ^- 2j ^+ 2k ^) and r⃗ = -4i ^- k ^+ Î¼(3i ^- 2j ^- 2k ^).**

**Solution**

The given lines are

r⃗ = 6i ^ + 2j ^ + 2k ^ + Î»(i ^- 2j ^+ 2k ^) **...(1) **

r⃗ = -4i ^ - k ^+ Î¼(3i ^ - 2j ^ - 2k ^) **...(2) **

It is known that the shortest distance between two lines, , is given by

Comparing to equations (1) and (2), we obtain

Substituting all the values in equation (1), we obtain

d= |-108/12| = 9

Therefore, the shortest distance between the two given lines is 9 units.

**10. Find the coordinates of the point where the line through (5, 1, 6) and (3, 4, 1) crosses the YZ-plane**

**Solution**

It is known that the equation of the line passing through the points, (*x*_{1}, *y*_{1}, *z*_{1}) and (*x*_{2}, *y*_{2}, *z*_{2}), is

The line passing through the points, (5, 1, 6) and (3, 4, 1), is given by,

⇒ x = 5 - 2k, y = 3k + 1, z = 6 - 5k

Any point on the line is of the form (5 - 2k, 3k + 1, 6 - 5k).

The equation of YZ - plane is x = 0

Since the line passes through YZ - plane,

5 -2k = 0

**11. Find the coordinates of the point where the line through (5, 1, 6) and (3, 4, 1) crosses the ZX − plane.**

**Solution**

It is known that the equation of the line passing through the points, (*x*_{1}, *y*_{1}, *z*_{1}) and (*x*_{2}, *y*_{2}, *z*_{2}), is

The line passing through the points, (5, 1, 6) and (3, 4, 1), is given by,

⇒ x = 5 - 2k, y = 3k + 1, z = 6 - 5k

Any point on the line is of the form (5 - 2k, 3k + 1, 6 - 5k).

Since the line passes through ZX - plane,

3k + 1 = 0

**12. Find the coordinates of the point where the line through (3, −4, −5) and (2, −3, 1) crosses the plane 2 x + y + z = 7).**

**Solution**

It is known that the equation of the line through the points, (*x*_{1}, *y*_{1}, *z*_{1}) and (*x*_{2}, *y*_{2}, *z*_{2}), is

Since the line passes through the points, (3, -4, -5) and (2, -3, 1) its equation is given by,

⇒ x = 3 - k, y = k - 4, z = 6k - 5

Therefore, any point on the line is of the form (3 - k, k - 4, 6k - 5).

This point lies on the plane, 2x + y + z = 7

Therefore, 2(3 - k) + (k - 4) + (6x - 5) = 7

⇒ 5k - 3 = 7

⇒ k = 2

Hence, the coordinates of the required point are (3, -2, 2 - 4, 6×2 - 5) i.e., (1, -2, 7).

**13. Find the equation of the plane passing through the point (−1, 3, 2) and perpendicular to each of the planes x + 2y + 3z = 5 and 3x + 3y + z = 0.**

**Solution**

The equation of the plane passing through the point (−1, 3, 2) is*a *(*x* + 1) + *b* (*y* − 3) + *c* (*z* − 2) = 0 **…(1)**

where, *a*, *b*, *c* are the direction ratios of normal to the plane.

It is known that two planes, a_{1}x + b_{1}y + c_{1}z + d_{1} = 0 and a_{2}x + b_{2}y + c_{2}z + d_{2} = 0 perpendicular , if *a*_{1}*a*_{2} + *b*_{1}*b*_{2} + *c*_{1}*c*_{2} = 0

Plane (1) is perpendicular to the plane, x + 2y + 3z = 5

∴ a.1 + b.2 + c.3 = 0

⇒ a + 2b + 3c = 0 **...(2) **

Also, plane (1) is perpendicular to the plane, 3x + 3y + z = 0

∴ a.3 + b.3 + c.1 = 0

⇒ 3a + 3b + c = 0 **...(3) **

From equations (2) and (3), we obtain

⇒ a = -7k , b = 8k, c = -3k

Substituting the values of a, b, and c in equation (1), we obtain

-7k(x + 1) + 8k(y - 3) - 3k(z - 2) = 0

⇒ (-7x - 7) + (8y - 24) - 3z + 6 = 0

⇒ -7x + 8y - 3z - 25 = 0

⇒ 7x - 8y + 3z + 25 = 0

This is the required equation of the plane.

**14. If the points (1, 1, p) and (−3, 0, 1) be equidistant from the plane r⃗ .(3i ^+ 4j ^- 12k ^) + 13 = 0 then find the value of p.**

**Solution**

The position vector through the point (1, 1, p) is a1⃗ = i ^ + j ^ + pk ^ .

Similarly, the position vector through the point (-3, 0, 1) is

a2⃗ = -4i ^+ k ^.

The equation of the given plane is r⃗.(3i ^ + 4j ^ - 12k ^) + 13 = 0

It is known that the perpendicular distance between a point whose position vector is a⃗ and the plane,

r⃗.N⃗ = d, is given by ,

Here, N⃗ = 3i ^ + 4j ^ - 12k ^ and d = -13

Therefore, the distance between the point (1, 1, p) and the given plane is

It is given that the distance between the required plane and the points, (1, 1, p) and (-3, 0, 1), is equal.

∴ D_{1} = D_{2}

⇒ 20 - 12p = 8 or -(20 -12p) = 8

⇒ 12p = 12 or 12p = 28

⇒ p = 1 or p = 7/3

**15. Find the equation of the plane passing through the line of intersection of the planes r⃗.(i ^+ j ^+ k ^) = 1 and r⃗.(2i ^+ 3j ^- k ^) + 4 = 0 and parallel to x-axis . **

**Solution**

The given planes are r⃗ .(i ^ + j ^ + k ^) = 1

⇒ r⃗ .(i ^ + j ^ + k ^) -1 = 0

r⃗.(2i ^ + 3j ^ - k ^) + 4 = 0

The equation of any plane passing through the line of intersection of these planes is

Its direction ratios are (2Î» + 1), (3Î» + 1), and (1 - Î»).

The required plane is parallel to x - axis. Therefore, its normal is perpendicular to x - axis.

The direction ratios of x - axis are 1, 0, and 0.

∴ 1.(2Î» + 1) + 0(3Î» + 1) + 0(1 - Î») = 0

⇒ 2Î» + 1 = 0

⇒ Î» = -1/2

Substituting Î» = -1/2 in equation (1), we obtain

Therefore, its Cartesian equation is y - 3z + 6 = 0

This is the equation of the required plane.

**16. If O be the origin and the coordinates of P be (1, 2, −3), then find the equation of the plane passing through P and perpendicular to OP.**

**Solution**

The coordinates of the points, O and P, are (0, 0, 0) and (1, 2, −3) respectively.

Therefore, the direction ratios of OP are (1−0) = 1, (2−0) = 2, and (−3 − 0) = −3

It is known that the equation of the plane passing through the point (*x*_{1}, *y*_{1}, *z*_{1}) is a(x - *x*_{1}) + b(y - *y*_{1}) + c(z - z_{1}) = 0 where, a, b, and c are the direction ratios of normal.

Here, the direction ratios of normal are 1, 2, and -3 and the point P is (1, 2, -3).

Thus, the equation of the required plane is

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

⇒ z + 2y - 3z - 14 = 0

**17. Find the equation of the plane which contains the line of intersection of the planes r⃗ .(i ^+ 2j ^+ k ^) - 4 = 0, r⃗.(2i ^ + j ^- k ^) + 5 = 0, and which is perpendicular to the plane r⃗.(5i ^+ 3j ^- 6k ^) +8 = 0. **

**Solution**

The equations of the given planes are

The equation of the plane passing through the line intersection of the plane given in equation (1) and equation (2) is

The plane in equation (3) is perpendicular to the plane, r⃗.(5i ^+ 3j ^ - 6k ^) + 8 = 0

∴ 5(2Î» + 1) + 3(Î»+ 2) - 6(3 - Î») = 0

⇒ 19Î» - 7 = 0

⇒ Î» = 7/19

Substituting Î» = 7/19 in equation (3), we obtain

This is the vector equation of the required plane.

The Cartesian equation of this plane can be obtained by substituting r⃗.(xi ^ + yj ^ + zk ^) in equation (3).

⇒ 33x + 45y + 50z - 41 = 0

**18. Find the distance of the point (−1, −5, −10) from the point of intersection of the line r⃗ = 2i ^ - j ^ + 2k ^+ Î»(3i ^+ 4j ^+ 2k ^) and the plane r⃗.(i ^- j ^+ k ^) = 5.**

**Solution**

The equation of the given line is

r⃗ = 2i ^ - j ^ + 2k ^ + Î»( 3i ^+ 4j ^+ 2k ^) **...(1) **

The equation of the given plane is

r⃗ .(i ^- j ^+ k ^) = 5 **...(2) **

Substituting the value of r⃗ from equation (1) in equation (2), we obtain

Substituting this value in equation (1), we obtain the equation of the line as r⃗ = 2i ^- j ^+ 2k ^

This means that the position vector of the point of intersection of the line and the plane is r⃗ = 2i ^- j ^+ 2k ^.

This shows that the point of intersection of the given line and plane is given by the coordinates, (2, −1, 2). The point is (−1, −5, −10).

The distance *d* between the points, (2, −1, 2) and (−1, −5, −10), is

**19. Find the vector equation of the line passing through (1, 2, 3) and parallel to the planes r⃗ .( i ^- j ^+ 2k ^) = 5 and r⃗ .( 3i ^+ j ^+ k ^) = 6.**

**Solution**

Let the required line be parallel to be parallel to vector b⃗ given by,

The position vector of the point (1, 2, 3) is a⃗ = i ^+ 2j ^+3k ^

The equation of line passing through (1, 2, 3) and parallel to b⃗ is given by,

The line in equation (1) and plane in equation (2) are parallel. Therefore, the normal to the plane of equation (2) and the given line are perpendicular.

Substituting the value of b⃗ in equation (1), we obtain

r⃗ = (i ^+2j ^+ 3k ^) + Î»(-3i ^+ 5j ^+ 4k ^)

This is the equation of the required line.

**20. Find the vector equation of the line passing through the point (1, 2, −4) and perpendicular to the two lines: (x - 8)/3 = (y + 19)/-16 = (z - 10)/7 and (x - 15)/3 = (y - 29)/8 = (z - 5)/-5**

**Solution**

Let the required line be parallel to the vector b⃗ given by,

∴ Direction ratios of b⃗ are 2, 3 and 6.

∴ b⃗ = 2i ^+ 3j ^+ 6k ^

Substituting b⃗ = 2i ^+ 3j ^+ 6k ^ in equation (1), we obtain

r⃗ = (i ^+ 2j ^- 4k ^) + Î»(2i ^+ 3j ^+ 6k ^)

This is the equation of the required line.

**21. Prove that if a plane has the intercepts a, b, c and is at a distance of P units from the origin, then 1/a^{2} + 1/b^{2} + 1/c^{2} = 1/p^{2}.**

**Solution**

The equation of a plane having intercepts a, b, c with x, y, and z axes respectively is given by,

x/a +y/b +z/c = 1 **...(1)**

The distance (p) of the plane from the origin is given by,

**22. Distance between the two planes: 2x + 3y + 4z = 4 and 4x + 6y + 8z = 12 is(A) 2 units(B) 4 units(C) 8 units (D) 2/√29 units **

**Solution**

The equations of the planes are

2x + 3y + 4z = 4 **...(1) **

4x + 6y + 8z = 12

⇒ 2x + 3y + 4z = 6 **...(2) **

It can be seen that the given planes are parallel .

It is known that the distance between two parallel planes, ax + by + cz = d_{1} and ax +by +cz = d_{2}, is given by,

Thus, the distance between the lines is 2/√29 units.

Hence, the correct answer is D.

**23. The planes: 2 x − y + 4z = 5 and 5x − 2.5y + 10z = 6 are**

(A) Perpendicular

(B) Parallel

(C) intersect y-axis

(C) passes through (0, 0, 5/4)

**Solution**

The equations of the planes are

2x - y + 4z = 5 **...(1) **

5x - 2.5y + 10z = 6 ** ...(2) **

It can be seen that,

Therefore, the given planes are parallel.

Hence, the correct answer is B.