圓形既面積?要用英文150或以上字數解釋(15點)

Circle
The area of a circle is found by multiplying the constant pi (π or 3.14) times the square of the radius. The radius is 1/2 of the diameter.
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Formula
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Area = π x r2
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• π (pi) = 3.14
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• r2 (radius squared) = r x r
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Example
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Area = π x r2
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• r = 6 ft
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Area = π x r2
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• = 3.14 x (6 x 6)
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• = 3.14 x 36
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• = 113 ft2

解釋圓形面積公式的來源?
F.2? 或是F.5以上?