Once we know the radius of a circle, we can calculate the length of the circle (circumference) and the area inside the circle (disc).
Circumference of a circle = 2 × π × radius Circumference of a circle = 2πr
A disc is the region in a plane inside a circle.
Area of a disc = π × radius × radius Area of a disc = πr2
These formulae appear in the formulae and tables booklet. Worked example 1
Taking π = 3·14, fi nd (i) the circumference and (ii) the area of the disc shown on the right which has a radius of 4·5 cm . Give your answer correct to two decimal places.
Solution 4.5 cm
radius
radius
Disc
Circle
Worked example 2 Find the area the disc by (i) taking π = 22
Solution
12.6 cm the right which has a diameter of 12·6 cm.
___ 7 (ii) in terms of π , shown on
If asked to leave answer ‘in terms of π′ , then you can just leave the π in the answer. Do not substitute any approximation for π .
Worked example 3 The area of the circle on the right is 6 724π mm 2
Find (i) the radius and (ii) the circumference of the disc, correct to two decimal places.
Solution
r
If no value for π is given in a question then use the π button on your calculator.
Finding the square root is the reverse of squaring.