Before I start writing about the diameter, radius, and circumference of a circle I think it would be better if we know what each one is.
Here is an image that may help you understand these 3 words.
Since this image doesn't show Pi I guess I should just tell you what Pi is. Pi otherwise known as 3.14 is a mathematical constant.
The circumference of a circle is slightly more than three times as long as its diameter. The exact ratio is called π/Pi/3.14.
how do you find the circumference of a circle?
Let's pretend that the diameter of a circle is 10cm and we need to find the circumference. Well the rule to find the circumference is diameter (D) x π (3.14). 10cm x 3.14= 31.4cm, so the circumference would be 31.4cm. Since we already know that the radius is 1/2 of the diameter the rule would be 2πR (2 x 3.14 x radius, in this case 2 x 3.14 x 5).
Area
A = π r2A = (π÷4) × D2
Example: What is the area of a circle with radius of 2cm ?
A = π × r2
A = π × 22
A = π × (2 × 2)
A = 3.14 x 4= 12.6 cm2(rounded)
If we try doing this with the diameter (4 cm) it would look like this:
A= (π÷4) × D2
A= 0.785 x 4 squared
A= 0.785 x 16cm
A= 12.6 cm2(rounded)
Area Compared to a Square or rectangle...
A circle has about 80% of the area of a similar-width square (π divided by 4= 0.785).
A = π × 22
A = π × (2 × 2)
A = 3.14 x 4= 12.6 cm2(rounded)
If we try doing this with the diameter (4 cm) it would look like this:
A= (π÷4) × D2
A= 0.785 x 4 squared
A= 0.785 x 16cm
A= 12.6 cm2(rounded)
Area Compared to a Square or rectangle...
A circle has about 80% of the area of a similar-width square (π divided by 4= 0.785).
How do you find the area of a circle?
To find the area of a circle we need to know the radius of the circle (keep in mind that if you get the diameter you just divide it by 2 and do the rest of the steps) Once you know the radius of a circle you multiply it by itself and then by π (. If you're given the circumference then you need to: divide the Circumference by π to get the diameter and then divide it by 2 to get the Radius and then complete the steps which have already been given.
You can also find the area of a circle by using a parallelogram.
A = base x height (formula for the area of a parallelogram)
A= 1/2 circumference(also known as Pi x r) x radius (the base of the parallelogram is equal to 1/2 the circumference)
A = 1/2 x (2 x pi x r) x r (Remember, C= 2 x pi x r)
A = pi x r x r
If you need a visual for the rules its right here:
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