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Sphere:
A sphere is a three-dimensional figure obtained by the revolution of a semicircle about its diameter as an axis.
 
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From the figure, ACB is a semicircle. If we rotate the semicircle ACB with diameter AB as an axis, we get the three-dimensional object sphere.
Properties of a sphere:
1. Sphere is made of all the points in the space which are equidistant from the fixed point called the centre.
 
2. Distance between the centre to the edge of the sphere is called the radius (r) of the sphere.
 
3. Distance between any two points on a sphere through the centre is called the diameter (d) of the sphere.
 
4. Diameter = 2 \ \times radius
Surface area of a sphere:
Surface area of a sphere = 4 \ \times Area of a circle
 
= 4 \times \pi r^2
Surface area of a sphere = 4 \pi r^2sq. units.