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imc

geometry intermediate

Problem

A dilation of the plane--that is, a size transformation with a positive scale factor--sends the circle of radius centered at to the circle of radius centered at . What distance does the origin , move under this transformation?
(A)
(B)
(C)
(D)
Solution
The center of dilation must lie on the line , which can be expressed as . Note that the center of dilation must have an -coordinate less than ; if the -coordinate were otherwise, then the circle under the transformation would not have an increased -coordinate in the coordinate plane. Also, the ratio of dilation must be equal to , which is the ratio of the radii of the circles. Thus, we are looking for a point such that (for the -coordinates), and . We do not have to include absolute value symbols because we know that the center of dilation has a lower -coordinate, and hence a lower -coordinate, from our reasoning above. Solving the two equations, we get and . This means that any point on the plane will dilate to the point , which means that the point dilates to . Thus, the origin moves units.
Final answer
C