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Q.If I am presented with a portion of a circle (an arc), how can I determine the size (diameter) of the circle that the arc is part of?

A.Martin Holladay responds: First, connect any two points of the arc with a straight line. Find the center of that line and draw a perpendicular line through the center point. Then do the same thing again elsewhere on the arc — connect two points with a straight line and draw a perpendicular line through the center of that line.

The point where the two perpendicular lines cross is the center of the circle.

Finding the Center of a Circle

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