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Answer:
scale factor = (image distance)/(pre-image distance)
Explanation:
The scale factor from a pre-image to an image is the ratio of some linear measurement of the image to the corresponding linear measurement of the pre-image:
scale factor = (image distance)/(pre-image distance)
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The corresponding measurements can be measurements of side length, arc length, diameter, circumference, perimeter, chord length, or the length of any point on the image to the center of dilation (if known). The only requirement is that corresponding measurements be used, and that they be linear dimensions, not areas or volumes or angles (or anything else with units other than distance units).