NCERT Solutions class 12 Maths Miscellaneous

NCERT Solutions class 12 Maths Miscellaneous Class 12 Maths book solutions are available in PDF format for free download. These ncert book chapter wise questions and answers are very helpful for CBSE board exam. CBSE recommends NCERT books and most of the questions in CBSE exam are asked from NCERT text books. Class 12 Maths chapter wise NCERT solution for Maths part 1 and Maths part 2 for all the chapters can be downloaded from our website and myCBSEguide mobile app for free.

Download NCERT solutions for Three Dimensional Geometry as PDF.

NCERT Solutions class 12 Maths Three Dimensional Geometry 

1.Show that the line joining the origin to the point (2, 1, 1) is perpendicular to the line determined by the points  

Ans. We know that direction ratios of the line joining the origin (0, 0, 0) to the point are

 = 2 – 0, 1 – 0, 1 – 0 = 2, 1, 1 =

Similarly, direction ratios of the line joining the points  and  are

 =  =  =

For these two lines,

=  = 2 – 2 + 0 = 0

Therefore, the two given lines are perpendicular to each other.

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NCERT Solutions class 12 Maths Miscellaneous

2.If  and  are the direction cosines of two mutually perpendicular lines, show that the direction cosines of the line perpendicular to both of these are  

Ans.  and  are direction cosines of two mutually perpendicular of two given lines L₁ and L₂. (say)

Let  and  be the unit vectors along these lines L₁ and L₂.

  and

Let L be the line perpendicular to both the lines L₁ and L₂ and let  be a unit vector along line L perpendicular both lines L₁ and L₂.

 Cross-product of two vectors =

[ L1  L2 (given,  angle between them is ]

 

Since, is a unit vector, therefore its components are its direction cosines.

Thus, direction cosines of  are

direction cosines of line L are

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NCERT Solutions class 12 Maths Miscellaneous

3.Find the angle between the lines whose direction ratios are  and  

Ans. Direction ratios of one line are

A vector along this line is

Direction ratios of second line are

A vector along second line is

Let  be the angle between the two lines, then

 =

=  = 0 =

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NCERT Solutions class 12 Maths Miscellaneous

4.Find the equation of the line parallel to axis and passing through the origin.

Ans. We know that a unit vector along axis is

 Direction cosines of axis are coefficients of  in the unit vector

i.e., 1, 0, 0 =

Equation of the required line passing through the origin (0, 0, 0) and parallel to axis is   

Vector equation of the required line is

 [ and  ]

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NCERT Solutions class 12 Maths Miscellaneous

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

Ans. Given: Points A B C and D

 Direction ratios of line AB are

A vector along the line AB is

Similarly, direction ratios of line CD are

A vector along the line AB is

Let  be the angle between the two lines, then

 = =  =  =  = 1

=

Therefore, lines AB and CD are parallel.

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NCERT Solutions class 12 Maths Miscellaneous

6.If the lines  and  are perpendicular, find the value of  

Ans. Given: Equation of one line is

Direction ratios of this line are its denominators, i.e.,  =

A vector along this line is

Again, equation of second line is

Direction ratios of this line are its denominators, i.e.,  =

A vector along this line is

Since these given lines are perpendicular.

  

 

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NCERT Solutions class 12 Maths Miscellaneous

7.Find the vector equation of the line passing through (1, 2, 3) and perpendicular to the plane  

Ans. The required line passes through the point P (1, 2, 3).

 Position vector  (say) of point P is (1, 2, 3)

 

Equation of the given plane is

Comparing with  

Since, the required line is perpendicular to the given plane, therefore, vector  along the required line is

 Equation of the required line is

 

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NCERT Solutions class 12 Maths Miscellaneous

8.Find the equation of the plane passing through  and parallel to the plane  

Ans. Equation of any plane parallel to the plane  is …..(i)

Plane (i) passes through

Putting  in eq. (i), we get

  

Putting the value of  in eq. (i), to get the required plane is

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NCERT Solutions class 12 Maths Miscellaneous

9.Find the shortest distance between lines  and  

Ans. Given: Vector equation of one line is

Comparing with  we get

 and

Again given: Vector equation of another line is

Comparing with  we get

 and

We know that length of shortest distance between two (skew) lines is  ..(i)

Now =  =

Again

Expanding along first row,

=

 

=

And =

Putting these values in eq. (i), length of shortest distance =

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NCERT Solutions class 12 Maths Miscellaneous

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

Ans. Given: A line through the points A (5, 1, 6) and B (3, 4, 1)

 Direction ratios of this line AB are

 3 – 5, 4 – 1, 1 – 6

Equation of the line AB is

 ……….(i)

Now we have to find the coordinates of the point where this line AB crosses the YZ-plane

i.e.,  ……….(ii)

Putting  in eq. (i), we get

  and

 and   and

 and

Thus, required point is P

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NCERT Solutions class 12 Maths Miscellaneous

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

Ans. Given: A line through the points A (5, 1, 6) and B (3, 4, 1)

 Direction ratios of this line AB are

3 – 5, 4 – 1, 1 – 6

Equation of the line AB is

 ……….(i)

Now we have to find the coordinates of the point where this line AB crosses the ZX-plane

i.e.,  ……….(ii)

Putting  in eq. (i), we get

  and

 and   and

 and

Thus, required point is P

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NCERT Solutions class 12 Maths Miscellaneous

12.Find the coordinates of the point where the line through  and  crosses the plane  

Ans. Direction ratios of the line joining the points A and B are

 

 Equation of the line AB are  ………(i)

Equation of the plane is  ………(ii)

Now to find the point where line (i) crosses plane (ii),

From eq. (i) (say)

  

 ……….(iii)

Putting the values of  in eq. (ii), we get

  

Putting  in eq. (iii), point of intersection of line (i) and plane (ii) is

Thus, required point of intersection is

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NCERT Solutions class 12 Maths Miscellaneous

13.Find the equation of the plane passing through the point  and perpendicular to each of the planes  and  

Ans. Since equation of any plane through the point  is

  ……….(i)

  

This required plane is perpendicular to the plane  

Product of coefficients  ……….(ii)

Again the required plane is perpendicular to the plane

Product of coefficients  ……….(iii)

Solving eq. (ii) and (iii), we get

Putting these values of  in eq. (i), we get

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NCERT Solutions class 12 Maths Miscellaneous

14. If the points  and  be equidistant from the plane  then find the value of  

Ans. Equation of the given plane is

[  = Position vector of any point  on the plane ]

 ……….(i)

Also, the point  and  are equidistant from plane (i)

(Perpendicular) distance of point  from plane (i)

= Distance of point  from plane (i)

 [ If  then]

Taking positive sign,

Taking negative sign,

Hence, the values of  are 1 or

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NCERT Solutions class 12 Maths Miscellaneous

15.Find the equation of the plane passing through the line of intersection of the planes  and  and parallel to axis.

Ans. Equation of one plane is

 ……….(i)

Equation of the second plane is ……….(ii)

Since, equation of any plane passing through the line intersection of these two planes is

L.H.S. of I +  (L.H.S. of II) = 0

 ……….(i)

Comparing  we have

Now required plane (i) is parallel to axis ( a vector  along axis is  = )

  

Putting  in eq. (i), the equation of required plane,

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NCERT Solutions class 12 Maths Miscellaneous

16.If O be the origin and the coordinates of P be  then find the equation of the plane passing through P and perpendicular to OP.

Ans. Given: Origin O (0, 0, 0) and point P

To find: Equation of the plane passing through P =

 Direction ratios of normal OP to the plane are

 

Equation of the required plane is

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NCERT Solutions class 12 Maths Miscellaneous

17.Find the equation of the plane which contains the line of intersection of the planes   and which is perpendicular to the plane

Ans. Equation of any plane passing through (or containing) the line of intersection of the planes  and  is L.H.S. of I +  (L.H.S. of II) = 0

 ……….(i)

Comparing with  we have,

Now plane (i) is perpendicular to the given plane

Comparing with  we have,

For perpendicular planes

Putting  in eq. (i), equation of required plane is

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NCERT Solutions class 12 Maths Miscellaneous

18.Find the distance of the point  from the point of intersection of the line  and the plane

Ans. Given: A point P (say)

and equation of the line  ……….(i)

equation of the plane is

Putting the value of  from eq. (i) in eq. (ii),

Putting  in eq. (i),

Therefore, Point of intersection is  =

 Distance of the given point P from the point of intersection is

=

=

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NCERT Solutions class 12 Maths Miscellaneous

19.Find the vector equation of the line passing through  and parallel to the plane  and

Ans. The required line passes through the point A (1, 2, 3) =

 = Position vector of point A =

Let  be any vector along the required line.

Vector equation of required line is

  ……….(i)

Since required line is parallel to the plane

 and

Comparing with  we have,

And Comparing with  we have,

Since  is perpendicular to both  and

 =

Expanding along first row,

 =

Putting this value of  in eq. (i), vector equation of required line,

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NCERT Solutions class 12 Maths Miscellaneous

20.Find the vector equation of the line passing through the point  and perpendicular to the two lines:  and  

Ans. Given: A point on the required line is A

 Position vector of point A is

Also given equations of two lines

 and

Direction ratios of given two lines are  and

Now =

Expanding along first row,

=  =

 

Equation of the required line is

Again replacing  by

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NCERT Solutions class 12 Maths Miscellaneous

21.Prove that if a plane has the intercepts  and is at a distance of  units from the origin, then  

Ans. We know that equation of plane making intercepts  (on the axes) is

Given: Perpendicular distance of the origin (0, 0, 0) from plane =

  =

 

Squaring both sides,

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NCERT Solutions class 12 Maths Miscellaneous

Choose the correct answer in Exercise Q. 22 and 23.

22.Distance between the two planes:  and  is

(A) 2 units(B) 4 units(C) 8 units(D)  units

Ans. Equation of one plane is  

Equation of second plane is  

Here

Since,  therefore, the given two lines are parallel.

We know that the distance of the parallel lines =

=

=

Therefore, option (D) is correct.

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NCERT Solutions class 12 Maths Miscellaneous

23. The planes:  and  are

(A) Perpendicular(B) Parallel

(C) intersect axis(D) passes through  

Ans. Equations of the given planes are  

and  

For perpendicular =  =

  

 Planes are not perpendicular.

For parallel

 given planes are parallel.

Therefore, option (B) is correct.

NCERT Solutions class 12 Maths Miscellaneous

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