ABSTRACT
Problem 1. Find the circumference of a circle of radius 12.0cm
Circumference,c = 2×π × radius = 2πr = 2π(12.0) = 75.40cm
Problem 2. If the diameter of a circle is 75mm, find its circumference
Circumference,c = π × diameter = πd = π(75) = 235.6mm
Problem 3. Determine the radius of a circular pond if its perimeter is 112m
Perimeter = circumference, c = 2πr
Hence, radius of pond, r = c 2π
= 112 2π
= 17.83cm
Problem 4. In Figure 26.4, AB is a tangent to the circle at B. If the circle radius is 40mm and AB = 150mm, calculate the length AO
A tangent to a circle is at right angles to a radius drawn from thepoint of contact; i.e.,ABO = 90◦.Hence, using Pythagoras’ theorem,
AO2 = AB2 + OB2
from which, AO = √
AB2 + OB2
= √
1502 + 402 = 155.2mm
Now try the following Practice Exercise
PracticeExercise 101 Properties of a circle (answers on page 351)
1. Calculate the length of the circumference of a circle of radius 7.2cm.
