I had been using Inkscape for several years before figuring out that I could use the Star/Polygon Tool for drawing triangles in Inkscape. I had previously either drawn them using straight lines, or by starting with a rectangle and then deleting nodes. Both methods always seemed a bit cumbersome to me and I was pretty excited when I figured out there is an easier way! Perhaps I am the only one out there that struggled with this, but I doubt it. So…. here’s a little”mini tutorial” for anyone that is looking for a solution to this issue.
First, click on the Star/Polygon Tool 
which is located on the Tool Box Bar on the left side of your screen. Now go up to your Tools Control Bar at the top of the screen. You’ll see in the screen capture below that I have selected Star. However, when making a triangle, it doesn’t matter whether it is set to Star or Polygon. The only thing you need to be concerned with is setting the Corners to 3.
Now you are ready to use the Star/Polygon Tool to draw your triangle. You will get a triangle with 3 equal sides and angles. The triangle will have one node that you can drag to change the position or size of the triangle. If you want to change the shape of your triangle, you’ll need to convert it to a Path.
Addendum: cleversomeday, over on the Inkscape Cutting Design forum has another great idea:
Thank you!!! Thank you!!
You are very welcome!
It really helped. Thank you.
So glad it helped you!
Thank you. This tutorial is great as the Inkscape . I have doing designs in raw tikz, so I can really appreciate the facilities provided by Inkscape.
Thank you.
very clear, so thanks
You’re welcome!
That is so simple… Never saw this toolbar :))) Thank you!
You are welcome!