The picture below illustrates the relationship between the radius, and the central angle in radians. The formula is
s = rθ where s represents the arc length, θ represents the central angle in radians and r is the length of the radius.

Demonstration of the Formual s=rθ

The interative demonstration below illustrates the relationship between the central angle of a circle, measured in radians, and the length of the intercepted arc. Just click and drag the points.

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What is the value of the arc length s in the circle pictured below?