Area Of Circle

The formula is:

Area of Circle = π r²

where
π, pi is a constant, approximately 3.14
r is the radius

Take care to use the Radius and not the Diameter in this rule.

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Example One - Contact Lens Size

A contact lens has a radius of 4 mm. What is its area?
(Assume the surface is flat and that π is 3.14.)

Answer:
Area of circle
= π r²
= 3.14 × (4 × 4)
= 50.24 mm²

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Example Two - Lotus Lily Leaf

Plant leaves use sunshine, carbon dioxide and water to photosynthesize and make oxygen and sugar. This lily leaf has a diameter of 30 cm. What is the area to be available for photosynthesis?
(Use π = 3.14.)

Answer:
Area of circle
= π r²
= 3.14 × (15 × 15)
= 706.5 cm²

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Example Three - Lampshade Fabric

A lampshade design is made of an annulus with a central angle of 160 degrees. The diameter of the smaller circle is 10 cm. The diameter of the larger circle is 20 cm. What area of fabric is used in the lampshade? (Use π = 3.14.)

Answer:
Area of larger circle
= π R²
= 3.14 × 10 × 10
= 314 cm²

Area of smaller circle
= π r²
= 3.14 × 5 × 5
= 78.5 cm²

Area of whole annulus
= 314 - 78.5
= 235.5 cm²

Area of 200 degrees of annulus
= (200° ÷ 360°) × 235.5
= 130.8 cm²