Math CentralQuandaries & Queries


Question from cynthia, a student:

Hi is this the correct formula for this problem? What is the approximate area of a circular garden that is 20 feet in diameters?

A=(20)= 10ft
A= pie (10ft)^2
A=(3.14) (100ft)^2

Hi Cynthia.

The formula for the area of a circle is one you should try to memorize:

A = π r2

where A is the area and r is the radius.

Sometimes you don't know the radius, but you do know the diameter. You should also know that 2r = d, where d is the diameter. That means the radius is half the diameter.

If you have a circular garden that is 20 feet in diameter, then its radius is 10 feet. That means r = 10, so

A = π × 102 = π × 100 ≈ 3.14 × 100 = 314 square feet.

Hope this helps,
Stephen La Rocque.

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