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Thread: Showing Construction Arcs

  1. #1
    Sketchpad Developer
    Join Date
    Oct 2010

    Showing Construction Arcs

    Over the years I've received several support questions like this one: "I would like to leave the remnants of construction lines in the vicinity of an intersection or point, without the entire loop of a circle."

    This question is a result of a desire to simulate the use of compass and straightedge on paper, where students used to make only small arcs, rather than complete circles, to show an intersection. Such marks are more useful on the fixed medium of paper than they are in Sketchpad, where the ability to drag can result in intersection points moving wildly about, likely to places where the small arcs would no longer intersect. In the dynamic environment of Sketchpad, if the intersection were based on small arcs, the entire construction would fail.

    If you still want such an arc for cosmetic purposes, you can have it, but you'll have to use a circle first so that the construction of the intersection is robust. Then rotate the intersection point in both directions, and construct the arc on the circle between the rotated points. That way the arcs will still include the intersection point no matter how the sketch is dragged.

    For the sake of convenience, I've attached a tool that does this.
    Attached Files

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