Shading the other side of an equality (2D)

Tom Chignell shared this question 1 month ago
Answered

I am teaching Linear Programming (in two variables) and our custom is to create a feasible region by shading out the area which is outside the feasible region. Most packages seem to shade inside as does GeoGebra. For example y>x on GeoGebra shades above the diagonal line (excluding the line) but I would like to shade below the line.

This is for year 13 students who are not that confident with Algebra so asking them to simply invert the inequality won't cut it. Is there a setting you can use to switch the shading?

At the moment, we are using Autograph V3.3 which shades as required by default but its not a great application. A browser based solution would be much better.

Thanks

Tom

Comments (5)

photo
1

as integrals, polygons etc inequalities are objects and by creating them they (=the regions that match the inequation) get a color. That's the logic behind the coloring. In the Style tab you can choose inverted filling to color the other side. However not as an automatic setting.

chris

photo
1

Thx Chris. I hadn't found this feature. But I do find it deals with the line a little confusingly. The logical inverse of y<x is y>=x so I would have expected inverse filling of this inequality to include the line (i.e. not be hashed).

Tom

photo
photo
1

or define ever ¬(...........) like command for inverting filling

or request inverse filling in preferences-->default-->inequality-->style in ver5 and settings default in ver6

or create a custom tool ie: inverse(y<x)

photo
1

Thx. The logical not function deals with the inequality (and therefore the shading and line) in the way I would expect (see my answer to Chris). Mind you, generating the ¬ symbol on MacOS is a challenge. I will have to do more research.

I have a Mac version 6 of GeoGebra downloaded from the App Store. I don't see setting default.

Thx

Tom

photo
1

settings default is in ver5 classic

photo
© 2019 International GeoGebra Institute