Drawing a trapezoid with known lengths, unknown angles

[Quik&Easy]

Using 2011 LT, given 4 dimensions on a sketch of a trapezoid but no angles off the base line or height of anything, can it be drawn with overlapping radii, similar to how one would draw a triangle with known lengths and nothing else?  To visualize, use a baseline horizontal of 40, left side 9.5, right side 28, top length 40.  I have a number of these to draw, with diminishing dimensions and some fairly radical changes of the bottom and top lengths; they will not always be equal. I've only been able to come close by trial and error, rotating bit by bit by fractions of degrees from a line drawn to tangent, etc.  It seems as though it should be doable but I'm not seeing the mechanics of it.

I believe I should ask the client for a starting angle to pin down one variable so I can then use overlapping radii. There is obviously a near infinite number of possibilities for the location of that top 40 line. 

[dlanorh]

The answer is No, not without a fixed angle, the height to one of the top angles or a diagonal.

[eldon]

The one feature of a trapezoid is that it has a pair of parallel sides. So using that knowledge, it may be that the shape that is required is a symmetrical truncated pyramid.

 

[BIGAL]

Nice one Eldon if you make the 28 the horizontal length then it is solvable 1/2 the 28 1/2 the 9.5 etc 

 

[eldon]

Like this (with a few construction lines and a circle)

 

 

[eldon]

Or even like this (with construction lines and a circle)

 

[Quik&Easy]

Thank you, I can see where it would work with 2 lines of the same length, in this case the 40 dimension. Most of the next ones have unequal longer sides; some just an inch or so different, some as large as a 3" difference. Perhaps polygon is a more accurate term for what I've been asked to draw. I have asked the client for some reference angles. With the 2 longer sides being unequal, I can't have 2 parallel lines as a constant.

[eldon]

Yes, the distinction between a polygon and a trapezium is of the utmost importance.

 

But you can have trapeziums with different length sides, but two sides must be parallel.

[lrm]

Although AutoCAD LT does not include the Parametric feature of full AutoCAD my understanding is that you can edit parametric dimensional constraints in AutoCAD LT.  Try editing the attached drawing's parametric dimensions.  The left and right side have a parallel constraint applied to them to ensure the result is a trapezoid.

 

trapezoid.dwg

[Quik&Easy]

I had to back save it using another program to Ver. 2010. My 2011LT wouldn't open it as it is a later version. I hope that didn't change the properties of the drawing too much.  The dimensions do not even show up on the revised version, however.

 

I'm sorry, but I have no idea of what constraints are and how they work. I am a VERY basic user, working in 2D only. However, when I highlight the left and right sides and click "delete constraints" from menu on the ribbon, I get a message in the command line of " All constraints will be removed from selected objects…2 found
No constraints to delete".  Same if I highlight the entire form.

 

When I highlight a single line and enter the properties bar and attempt to change that length, that is unsuccessful as well.

My client will provide some further angle measurements if this project develops further. This is an artistic application for the end product so I'll have some leeway in the final drawing to tweek things. I did something similar a few years ago but had a starting angle of at least one line.

Edited October 25, 2018 by Quik&Easy
forgot to mention dimensions missing

[lrm]

OK, since parametric constraints won't work for you with AutoCAD LT here's a geometric construction procedure that should.

 

Assume you have 4 lines with the lengths of 10, 15, 20, and 35.  Let's also assume that the 20 and 35 lines are the parallel sides (figureA).

Step 1 (figure B) - create a circle of radius 15 about the end of line 35 (red) and make a copy 20 to the right (green).

Step 2 (figure C) - create a circle of radius 10 centered at the right end of 35 (yellow).

Step 3 (figure D) - create a line 20 long from the intersection of the yellow and green circles to the left and then add the other lines as noted.