The Geometer’s Sketchpad
®
for TI-89, TI-92 Plus, and
© 2000-2002 KCP Technologies, Inc.
28
Voyage™ 200 PLT User Guide and Reference Manual
The Compass tool requires two points to construct a new circle. These
points may already exist or can be created as the tool is used. The first
point defines the center of a circle. The second point defines the radius of
the circle.
Steps
Display
1. Make sure the Compass tool is
the active tool. If not, see
Choosing a New Tool (page 22).
2. Using the
@
cursor controls, move the Compass tool to where you’d
like the circle’s center to be (at an existing point, on a path, in a blank
area of the sketch, or at the intersection of two objects). Then press
¸
.
3. Use the
@
cursor controls to
move away from the center
point. As you move, Sketchpad
displays a circle growing from
the center.
4. Position the Compass tool where you’d like the point defining the
circle’s radius to be (again, at an existing point, on a path, in a blank
area of the sketch, or at the intersection of two objects). Press
¸
a second time.
5. Sketchpad constructs and
displays a circle centered at the
first point, passing through the
second point.
As mentioned in steps 2 and 4, the Compass tool may be positioned over
an existing point, over blank space, over a path, or over the intersection of
two objects. If the Compass is positioned over an existing point, Sketchpad
uses that point to define the circle’s center or radius point, as the case may
be (in other words, it doesn’t construct a new point there). If the Compass
is positioned over a path, Sketchpad constructs a point on that path object.
If the Compass is positioned in blank space, Sketchpad creates a new point
there when you press
¸
. And if the Compass is positioned over the
intersection of two objects, Sketchpad constructs the point of intersection.
Note that if you construct a circle so that its circumference appears to pass
through an existing point, but the radius (second) point was constructed
elsewhere, then when you drag the circle, it will no longer pass through the
Compass Tool
Use the Compass tool to construct circles in your sketch. The
Compass draws circles from a center point to a second point
on the circle’s circumference.
Constructing
Circles
The shortcut for activating
the Compass tool is
¥…
(TI-89:
¥
3
).
A Circle’s Defining
Points
Geometrically speaking,
Sketchpad’s Compass tool
functions as a collapsible
compass, meaning that it
doesn’t retain its radius
between uses.
