I have a triangle ABC. I need to draw the three normals KO, LO, MO from the incentre O to sides of the triangle and then extend they outside of sides (blue lines on figure below). I know how to draw the incentre O (Extensions → Render → Draw from triangle → Incentre).

I have found the expressions for the segments AK = AM, BK = BL, CL = CM through the sides AB, AC, and BC:
AK=AM=(AB+AC-BC)/2,
BK=BL=(AB+BC-AC)/2,
CL=CM=(BC+AC-AB)/2.

Question. Could you please give me an idea how to calculate the positions K, M, and L?
Can I do this on the page Extensions → Render → Draw from triangle →Custom Points and Options with build-in expressions?

Hi.

You are asking for calculations but refer to the extensions - do you need to calculate the points or just draw the normals?

Extensions - Draw From Triangle - Contact Triangle - will give you the points.

Easy to draw the normals using snapping.

ragstian, thanks for the reply.

do you need to calculate the points or just draw the normals?

I need draw the normals from the incentre automatically. I'm newby in Inkscape Extension and I posted my question here because I hope that it's not difficult to add the option "Normals from the incentre" on page Custom Points and Options.

That level of geometry and math is over my head. But the middle tab of that dialog looks like it probably makes the extension very flexible, if you know the right formula or equation or whatever it's called. It looks like it's meant to find specific points like that. I used to know about sin and cosine, but by now, I'd have to look them up

If you're asking for a new feature to be added to an existing extension, that would require someone who knows how to write extensions, and further, knows enough about this particular geometry (or math, or whatever).

Typically, new features are requested by filing a bug report (yeah, I know, but it will be tagged as a wishlist item). https://launchpad.net/inkscape

If someone in this forum happens to have the right skills, and the time to do it, it's theoretically possible it could happen fairly quickly. I've seen it happen. But not very often.

But I agree with you, that it looks like that middle tab can probably do it. You just need the right....formula, equation, expression...(oh my old brain!)...whatever.