- Perimeter of a semi circle
Perimeter of a semi circle = Half of the circumference of the circle + Twice the radius
= π r + 2r units
Note : The ratio of the circumference to the diameter of any circle is a constant.
The constant is denoted by π.
Once the child is provided a paper cutting of a semi circle then he/she will be able to
understand that the circumference of the semicircle is equal to half the circumference
of the circle plus twice the radius of the circle (which is nothing but the diameter).
- The ratio between the circumference and diameter of a circle is constant
CircumfereDiametencer = π
Therefore, Circumference = π × diameter
The idea may be explained orally with the assistance of necessary text material in
Braille.
- Concentric circles
Two or more circles having the same centre but different radii are said to be concentric
circles.
Note : Consider two concentric circles with the same centre ‘O’ and different radii. Let ‘R’
be the radius of the outer circle and ‘r’ be the radius of the inner circle. Let the distance
between the circumference of the two circles, the width be ‘w’. Then the relation between
the three units R, r and w is given by,
R = w + r
r = R – w
w = R - r
R
r
W
