# Intersection of two lines - help!

Sarah Harrelson shared this question 7 months ago

I'm trying to build my own version of an intersect three planes document. When three planes intersect in a line though, my intersection command seems to break. I have defined three planes named, p, q, and r. Then I defined two lines of intersection as:

Intersect(p,q)

Intersect(q,r)

This produces two lines named g and h respectively. When the intersection of the three planes is a point, the command

Intersect(g,h)

correctly produces the point of intersection. But when the intersection of the three planes is a line, then the command

Intersect(g,h)

outputs nothing.

I've attached my file, since I know I'm not communicating this well. I'm pretty low skilled here, so I apologize in advance if this question is easily remedied.

Thanks!

p.s. I am using Geogebra Classic 5 on my laptop. The output of a particular command has to be the same type - so it can't change between Point and Line.

What you can do is use Condition to Show Object to handle the different cases and show a point or line as appropriate.

So you can check if g and h are parallel (and also if they are equal) 1

"normal" les 2 droites g et h sont confondues leur "intersection" est une droite

je vous propose de faire apparaître un texte dans ce cas 2

The output of a particular command has to be the same type - so it can't change between Point and Line.

What you can do is use Condition to Show Object to handle the different cases and show a point or line as appropriate.

So you can check if g and h are parallel (and also if they are equal) 1

ce n'est pas une question de changer de type ici

Intersection pour 2 droites ne sait que renvoyer un point, si les droites sont confondues ou strictement parallèles la commande renvoie Point = ? 1

That's exactly the error I was running into Noel. I used Michael's Condition to Show Object suggestion to get around this. I only show one of the lines under the condition that the two lines are equal. In that case the "intersect" command creates a point that doesn't exist and so doesn't display. In the case the two lines intersect at a point, neither line is displayed but the point shows up. It feels like a clumsy solution, but it's working so thank you both! 2

Condition to Show Object c'est aussi ce que j'ai utilisé pour le texte  1

Thank you both so much! This was very helpful!