Arc (geometry)

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A circular sector is shaded in green with length L along the circle's perimeter
A circular sector is shaded in green with length L along the circle's perimeter

In Euclidean geometry, a circular arc is a closed segment of a differentiable curve in the two-dimensional plane; for example, an arc is a segment of a circle. If the arc segment occupies a great circle (or great ellipse), it is considered a great-arc segment.

The length of an arc of a circle with radius r and subtending an angle \theta\,\! (measured in radians) with the circle center—i.e., the central angle—equals \theta r\,\!. This is because

\frac{L}{\mathrm{circumference}}=\frac{\theta}{2\pi}.\,\!

Substituting in the circumference

\frac{L}{2\pi r}=\frac{\theta}{2\pi},\,\!

and solving for arc length, L, in terms of \theta\,\! yields

L=\theta r.\,\!

For an angle α measured in degrees, the size in radians is given by

\theta=\frac{\alpha}{180}\pi,\,\!

and so the arc length equals then

L=\frac{\alpha\pi r}{180}.\,\!

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