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Answer:
1/3
Explanation:
The scale factor is the ratio of image lengths to original lengths.
B'C' = 1 unit
BC = 3 units
The scale factor is B'C'/BC = 1/3.
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Additional comment
The center of dilation in this figure is 3 units below D' on the same vertical line. (It is 1 unit off the bottom of the grid shown.)