How do you find the arc length in inscribed angles? without knowing the radius

You can find arc length if you know the circumference and the central angle.
If the the central angle is PI radians then the arc is one half of the circumference. 
central angle = PI/2 radians, arc length = 1/4 circumference
central angle = PI/4 radians, arc length = 1/8 circumference, etc.

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