http://www.math.washington.edu/~king/coursedir/m444a01/lab/lab-12-5.html

How to dilate by a Marked Ratio

A.  Ratio defined by 3 points

In Geometer's Sketchpad, draw a polygon, say a quadrilateral ABCD.  Also draw a point O.  Construct the line OA and a point A' on this line.  The first button in the GSP applet at right does this. 

Now select, in order (be careful here - you can hold down the SHIFT key, if it helps),  points O, A, and A'.  Then select "Mark Ratio" from the Transform menu.

Also, holding down the shift key with the same points selected, in the same order, choose Ratio from the Measure menu.  This will show the ratio that you have marked on the screen.  It is not necessary but is informative. This is the second button.

Now drag A' up and down the line so that you understand what ratio you have measured.  In particular, what is the ratio when A' is located at O, at A, at the midpoint of OA.  When is the ratio –1?  When is the ratio 2?

Now, you can use the marked ratio to dilate the quadrilateral. 

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DilateByMarkedRatio