Circumference = diameter * pi
20 = diameter * pi
20/pi = diameter
(20/pi)/2 = radius
10/pi = radius
Area = (radius)2 * pi
Area = (10/pi)2 * pi
Area = 100/pi2 * pi
Area = 100/pi
Area ≈ 31.83 square units
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You dived the area by the circumference of the circle.
Circumference of a circle is (pi * diameter). Area of a circle is (pi * r2).
Area = (Circumference squared)/4 pi Circumference = 2 sqrt(Area times pi)
The area of a circle with circumference 9.42in is: about 7.061 square inches.
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)