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
Related topics:
Approximate the area of a circle
Learn to approximate the area of a circle
Area of a sector
Learn to compute the area of a sector

May 21, 17 09:19 PM
Sharpen your math skills by grade here with crystal clear lessons
Read More
New math lessons
Your email is safe with us. We will only use it to inform you about new math lessons.