ABSTRACT
Problem 3. Determine the radius of a circle if its perimeter is 112 cm
Perimeter D circumference, c D 2r Hence radius r D c
2 D 112
2 D 17.83 cm
Problem 4. In Fig. 18.4, AB is a tangent to the circle at B. If the circle radius is 40 mm and AB D 150 mm, calculate the length AO
A tangent to a circle is at right angles to a radius drawn from the point of contact, i.e. ABO D 90°. Hence, using Pythagoras’ theorem:
AO2 D AB2 C OB2 from which, AO D
√ AB2 C OB2
D √
1502 C 402 D 155.2 mm
Now try the following exercise
Exercise 65 Further problems on properties of a circle
1. Calculate the length of the circumference of a circle of radius 7.2 cm. [45.24 cm]
2. If the diameter of a circle is 82.6 mm, calculate the circumference of the circle.
