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This is a GeoGebra mini-tip for the command IsDefined, which is a Boolean or true/false
command that tells us whether an object exists (is defined) or not. So what is the easiest
way to see this? Let us draw 2 line segments A B and at this
moment we will have them cross C D. So they have an intersection point here. So let us
get the intersection point E. What happens if we make it so that the segments
do not intersect. That is, we will click and drag D off and we see that it says that E
is undefined. Drag it back. E is defined. So how can we make a Boolean for that so that
we can use that fact of whether E is defined or not.
Click down in the input bar and start typing “is” and the first command that comes
up is IsDefined. So click on it. And now we just type in the letter “capital E” and
hit enter. And momentarily c=true because E is defined. But if we click and drag D off,
c is now false. So this Boolean says “yes” E is now defined
or true. Or “no” E is not defined or false. That is it!