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円周 定義

Circumference may be defined by some as the distance around the outside of an arbitrary closed object (sometimes restricted to a closed curved object). However, the term perimeter is used for this purpose and circumference is restricted to mean the perimeter of a circle. Circumference is a complete circular arc. Circumference is also sometimes used to refer to the enclosing boundary itself of a curved lamina or disk. For radius r or diameter d = 2r, and C = 2 π r = π d, where π is pi.