Learning Illustrator, need help making a Penrose triangle

[gascenso]

Hi everyone! I'm a complete beginner in Illustrator and I am following the Adobe Illustrator CC - Essentials Training Course on Udemy. In one of the lectures he goes over how to use the shape builder tool to draw a Penrose ("impossible") triangle starting from a bunch of lines. I followed his exact steps, but the result came out different. Below you can see the setup: a bunch of lines with extra bits sticking out, which we want to delete.

I hit ctrl+a to select everything, then with the shape builder I delete the bits sticking out, but the result is the image below, which has the pointy bit missing at the top.

I cannot for the life of me figure out what I'm doing wrong. The "Gap detection" feature of the shape builder is turned on and set to Large.

Any help appreciated. Thanks!

[Ton Frederiks]

The lines at the top are not connected, use the direct selection tool (A) to drag a selection rectangle around the top and choose Object > Path > Join

[gascenso]

It didn't work when I just drag selected a rectangle around the top (gave me an error message along the lines of "you need to select two different paths to join them"), but then I made one of the segments a little shorter and noticed that the anchor points were indeed not connected (although if I didn't make them shorter, the anchor points appeared to overlap entirely, so much so that I did not originally notice the segments were disconnected). Then I did Object > Path > Join as you suggested, and it worked, thanks! So follow up question: since the anchor points were not connected, shouldn't the Gap Detection function of the shape builder tool fix the issue? Why do I manually have to do Object > Path > Join?

[Ton Frederiks]

I think, because the two anchorpoints are on top of each other, there is no gap to be detected when you. remove the extranous lines. But when you fill that area with the Shape Builder tool, it will need to join them in order to create a fillable area.