11th NCERT/CBSE Introduction to Three Dimensional Geometry Exercise 12.1 Questions 4
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Question (1)

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

Solution

If a point is on the x-axis, then its y-coordinates and z-coordinates are zero.

Question (2)

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

Solution

If a point is in the XZ plane, then its y-coordinate is zero.

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)

Solution

The x-coordinate, y-coordinate, and z-coordinate of point (1, 2, 3) are all positive. Therefore, this point lies in octant I.
The x-coordinate, y-coordinate, and z-coordinate of point (4, –2, 3) are positive, negative, and positive respectively. Therefore, this point lies in octant IV.
The x-coordinate, y-coordinate, and z-coordinate of point (4, –2, –5) are positive, negative, and negative respectively. Therefore, this point lies in octant VIII.
The x-coordinate, y-coordinate, and z-coordinate of point (4, 2, –5) are positive, positive, and negative respectively. Therefore, this point lies in octant V.
The x-coordinate, y-coordinate, and z-coordinate of point (–4, 2, –5) are negative, positive, and negative respectively. Therefore, this point lies in octant VI.
The x-coordinate, y-coordinate, and z-coordinate of point (–4, 2, 5) are negative, positive, and positive respectively. Therefore, this point lies in octant II.
The x-coordinate, y-coordinate, and z-coordinate of point (–3, –1, 6) are negative, negative, and positive respectively. Therefore, this point lies in octant III.
The x-coordinate, y-coordinate, and z-coordinate of point (2, –4, –7) are positive, negative, and negative respectively. Therefore, this point lies in octant VIII.

Question (4)

Fill in the blanks:

Solution

(i) The x-axis and y-axis taken together determine a plane known as xy-plane .
(ii) The coordinates of points in the XY-plane are of the form (x, y, 0) .
(iii) Coordinate planes divide the space into eight octants.
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⇒12.2