Modeling a uneven triangle/square: linesize known, degrees unknown

Hi there,
I need to model a triangle with the following 3 side-lengths: 420cm, 150cm and 400cm.

I do not have the degrees of the corners but I know none is 90 degrees.

How can I model this triangle with just the 3 lines so SketchUp calculates the degrees?
I think I should draw the 3 lines, connect them and then merge/weld the 1st and last corner ‘in center’ of these 2, but i can’t see how.

Moveing and rotating the lines changes their lengths, which is no good.

I can calculate the degrees using an online tool, but I assume SketchUP is Pro enough to do so for me :wink:

The same goes for a square i need to create: 40x56x41x30.

I can draw the 4 lines (connected) then I need to connect the first and the last vertice (point) without changing the line-sizes. Ie. SU should rotate/move the corners in a way the dimensio stay the same.

Is there some sort of slide command that connects the 1st and last corner without changing the line-sizes? (just the skewing).

There are no doubt other ways, but my Circle Intersect extension (available from the SketchUcation plugin store) was designed to find the exact intersection points of circles to support triangulation problems such as this. Draw one edge - say the 420cm - as a baseline, draw a 150 cm circle at one end and a 400cm circle at the other end. Then use the extension to create guide points at the exact intersections of the circles (the real ones, not the segmented SketchUp representation). Erase the no-longer-needed circles and draw lines from the ends of the baseline to the guide point. If you wish, then erase the guides.

1 Like

Thank you so far. That’s apretty clever way of doing it :wink:

For the triangle this will work (i just used a circle with 400 sides in order to get the snap-point (not in the position to install an extension right now).

This won’t work for the rectangle (4 sided polygon) though. Any further ideas?

Gotta review my high-school geometry! I think you can distort a 4-sided polygon without changing the length of the sides? If that’s right, the 4-sided problem doesn’t have a unique solution without at least one angle or diagonal.


The polygon has uneven corners, a bit like this:

Just with the mentioned dimension 40x56x41x30.

So, the idea is I draw a horizontal line of 40, vertical of 56, horizontal of 41 and vertical of 30.
Then I want to drag the last endpoint to the startpoint until they snap. When dragging this endpoint the other corners should move into position to keep the line-sizes as mentioned.

depends which ones…


It might be worth trying a simple experiment: cut four thin strips of paper the lengths of the sides. Place them so their ends touch. Then try rotating one side while keeping all the ends touching - it is possible! There is no unique polygon that uses those four sides as lengths. So, for example, in your figure you would need the length of the diagonal BD to make the shape rigid (and reduce it to a triangulation problem).

Edit: I rest my case:

John, thank you.
That was actually the thing I did before but didn’t work. I tried it again after your example and now it works!
Must have been doing something wrong.

Thx alot! Triangle part solved :wink:

Baumgartner, I now see what you mean. Silly me and thank you for the great example.

Now, let’s say I do get a diagonal, you mention to reduce it then to a triangulation problem.
Do you mean I then should divide the 4-sided poly in 2 triangles and use John his sollution?

Yes, or mine.

Or yours, absolutely. Thank you!

Back with a final question, i hope :wink:

The sollution provided by SLB and John work, i just can’t get the lenght right.
When rotating a line (as seen in Johns example) I can’t get the other line (smallest one in the example) to get the right length. The lenght ‘snaps’ at weird intervals instead of cm’s.

I tried different snapping options in de model-info window, and even disable snapping but i guess this controls the line I’m rotating and NOT the line i want to change into the correct length. Is there a way to do so?
(I though disable snapping would work,but no :blush: -( )

I thought I found out by setting the precision to 0cm (no decimals) and disable snapping.
I can now set the length to cm’s.
BUT, when i add decimals again, I see the cm’s are completed to round numbers…sigh

So, the questions still stands, how can i rotate 1 line and change the other line to a certain lenght with ROUND numbers?

Since this is kind of a floow-up question i’ll make a new post. Moderators, you can close this one :wink:

To add gas to the fire; Like many if not most SU uses IEEE 754 float to approximate real number and the accuracy is a trade off of range of numbers you can use and the accuracy and as such what is going on under the hood is different than what you may think.
You should follow what Dave suggested unless there is some over whelming reason you need the accuracy you cite.
For your info they are part of EMORY university. They have changed the IP so srearch for IEEE 754 format when you get there
Try the square root of (3).

This topic was automatically closed 91 days after the last reply. New replies are no longer allowed.