You dived the area by the circumference of the circle.
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We know that the area of a circle is pi * r2. We also know that the circumference of a circle is pi * d. Therefore, when you know the circumference of a circle and want to know the area, divide the circumference by pi to give you the diameter of the circle, divide the diameter by 2 to give you the radius, square the radius and then multiply by pi to get the area. So, 10/pi = 3.183. 3.183/2 = 1.5915. 1.59152 = 2.533 2.533 * pi = 7.958 to 3 decimal places. The area of your circle is therefore 7.958cm2.
Divide the known circumference by pi to find the diameter of the circle.
The great circle has circumference 40pi and we know circumference is 2pixr so 20 is the radius of the sphere. Now use this and the formula that surface area is 4pir^2 so the surface are is 4pi(20)^2. or pi(1600)
The circumference of a circle that has a radius of 5 inches is 10 X pi or approximatly 31.415926535897932384626433832795. If you know the radius you can find the circumference by multiplying it by 2(pi)
If you know the diameter and need the circumference you times the diameter of the circle by 3.14 also known as pie. That is how you get the circumference by only knowing the diameter. :)