Circle

Circumference of a circle
$$C = 2\cdot \pi\cdot R$$
R - radius of a circle
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      =   
Calculate 'C'

Circumference of a circle (by diameter)
$$C = \pi\cdot d$$
d - diameter of a circle
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      =   
Calculate 'C'

Length of an arc of a circle‎
$$l = \frac{\pi\cdot R\cdot \alpha}{180}$$
R - radius of a circle
α - central angle
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      =   
Calculate 'l'

Length of a chord of a circle‎
$$AB = 2\cdot R\cdot sin(\alpha)$$
R - radius of a circle
α - central angle
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      =   
Calculate 'AB'

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