Area of a Circle
The formula for the area of a circle is:
A = pi × r
^{2}
If r is known, simply substitute the known value for r in the formula. Use 3.14 for pi.
Example #1:
Calculate A if r = 4 inches
A = pi × r
^{2} = 3.14 × 4
^{2} = 3.14 ×16 = 50.24 inches
^{2}
Notice that the unit (inches) is squared to show that it was multiplied by itself twice
Example #2:
Calculate A if r = 6 inches
A = pi × r
^{2} = 3.14 × 6
^{2} = 3.14 × 36 = 103.04 inches
^{2}
Examples #3
If D = 20 cm, calculate the area.
There is a couple of ways to solve this. The first way is to get r and then replace its value
Since r is half the diameter, r = 20 divided by 2
r = 10 cm
A = pi × r
^{2} = 3.14 × 10
^{2} = 3.14 × 100 = 314 cm
^{2}
The other way to solve this is to use the diameter.
We will need to express the formula in terms of the diameter.
The radius is half the diameter
Replace r into the formula A = πr
^{2}.
Notice that r
^{2} = r × r
Notice that r
^{2} = r × r
Let's us use the formula above now to see if we get the same answer.
A = 3.14 × 100
A = 314 cm
^{2}
Examples #4
If A = 78.5 inches
^{2}, find r
Review
solving multiplication equations
before looking at the solution below.
r = 5 inches because 5
^{2} = 5 × 5 = 25
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