# Class 11 Maths NCERT Solutions For Chapter 12 Introduction to Three Dimensional Geometry

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## In this pdf file you can see answers of following Questions

### EXERCISE 12.1

Question 1. A point is on the x-axis. What are its y-coordinate and z-coordinates?

Question 2.A point is in the XZ-plane. What can you say about its y-coordinate?

Question 3.Name the octants in which the following points lie:
(1,. 2, 3), (4, –2, 3), (4, –2, –5), (4, 2, –5), (– 4, 2, –5), (– 4, 2, 5), (–3, –1, 6) (2, – 4, –7).

Question 4.Fill in the blanks:
(i) The x-axis and y-axis taken together determine a plane known as_______.
(ii) The coordinates of points in the XY-plane are of the form _______.
(iii) Coordinate planes divide the space into ______ octants.

### EXERCISE 12.2

Question 1. Find the distance between the following pairs of points:
(i) (2, 3, 5) and (4, 3, 1) (ii) (–3, 7, 2) and (2, 4, –1)
(iii) (–1, 3, – 4) and (1, –3, 4) (iv) (2, –1, 3) and (–2, 1, 3).

Question 2.Show that the points (–2, 3, 5), (1, 2, 3) and (7, 0, –1) are collinear.

Question 3.Verify the following:
(i) (0, 7, –10), (1, 6, – 6) and (4, 9, – 6) are the vertices of an isosceles triangle.
(ii) (0, 7, 10), (–1, 6, 6) and (– 4, 9, 6) are the vertices of a right angled triangle.
(iii) (–1, 2, 1), (1, –2, 5), (4, –7, 8) and (2, –3, 4) are the vertices of a parallelogram

Question 4.Find the equation of the set of points which are equidistant from the points (1, 2, 3) and (3, 2, –1).

Question 5.Find the equation of the set of points P, the sum of whose distances from A (4, 0, 0) and B (– 4, 0, 0) is equal to 10.

### EXERCISE 12.3

Question 1. Find the coordinates of the point which divides the line segment joining the points (– 2, 3, 5) and (1, – 4, 6) in the ratio (i) 2 : 3 internally, (ii) 2 : 3 externally.

Question 2.Given that P (3, 2, – 4), Q (5, 4, – 6) and R (9, 8, –10) are collinear. Find the ratio in which Q divides PR.

Question 3.Find the ratio in which the YZ-plane divides the line segment formed by joining the points (–2, 4, 7) and (3, –5, 8).

Question 4.Using section formula, show that the points A (2, –3, 4), B (–1, 2, 1) and C 0 1 2 3 are collinear.

Question 5.Find the coordinates of the points which trisect the line segment joining the points P (4, 2, – 6) and Q (10, –16, 6).

### Miscellaneous Exercise on Chapter 12

Question 1. Three vertices of a parallelogram ABCD are A(3, – 1, 2), B (1, 2, – 4) and C (– 1, 1, 2). Find the coordinates of the fourth vertex.

Question 2.Find the lengths of the medians of the triangle with vertices A (0, 0, 6), B (0,4, 0) and (6, 0, 0).

Question 3.If the origin is the centroid of the triangle PQR with vertices P (2a, 2, 6), Q (– 4, 3b, –10) and R(8, 14, 2c), then find the values of a, b and c.

Question 4.Find the coordinates of a point on y-axis which are at a distance of 5 2 from the point P (3, –2, 5).

Question 5.A point R with x-coordinate 4 lies on the line segment joining the points P(2, –3, 4) and Q (8, 0, 10). Find the coordinates of the point R. [Hint Suppose R divides PQ in the ratio k :1.The coordinates of the point R are given by , 10 4 1 , 3 1 8 2 k k k k k ]. .

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