The commonly used formula is:
Circumference = 2丌r
丌 is a constant. Usually, it is usually taken as approximately 3.14 or 22//7.
Your calculator gives 丌 as a decimal up to many places, but we don't usually need this level of accuracy.
Area of circle = 丌r²
Example1:
Find the area of the given circle which has a radius of 14 cm. (Take 丌 as 22/7)
Area of circle = 丌r²
= 22/7 x 14 x 14
= 616 cm² Ans: 616 cm²
Finding radius of a circle when you are given the area:
Example2:
Find the radius of a circle which has an area of 1386 cm². (Take 丌 as 22/7)
Area of circle = 丌r²
1386 = 22/7 x r²
r² = 1386 x 7/22
= 441 = 21 x 21
r = 21
Finding the radius of a circle when you are given the circumference:
Example3:
Find the radius of a circle which has a circumference of 22 cm. (Take 丌 as 22/7)
Circumference of circle = 2丌r
22 = 2 x 22/7 x r
r = 22 x 1/2 x 7/22
= 7/2
= 3½ cm Ans: 3½ cm
Example4:
Find the radius of a circle which has a circumference of 50 cm. (Take 丌 as 3.14)
Give your answer as a decimal with 2 places.
Circumference of circle = 2丌r
50 = 2 x 3.14 x r
r = 50 ፥ 2 ፥ 3.14
= 7.96178...
≈ 7.96 cm Ans: 7.96 cm
