Radius Dimension
The Radius Dimension 
tool measures the
radius of an arc and places a radial dimension.
Creating a radius dimension
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Select the Radius Dimension tool.
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Pick an arc segment.
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Move the mouse to position the dimension, then click to place it. Hold Shift to include the center mark.
The Text and Tolerance input boxes let you override the dimension text and add a tolerance. See the section overview for the options shared by all dimension tools.