# Math Help - Trapezium question

1. ## Trapezium question

Just wondering if someone can help me with a work problem please.

From the picture, if I only know the 6 lengths of a,b,c,d,e, & f, (the 6 vertical lengths) how can I calculate AB & CD?

Thanks

2. I think you need more than that. How about the widths?

3. Originally Posted by emersong
Just wondering if someone can help me with a work problem please.

From the picture, if I only know the 6 lengths of a,b,c,d,e, & f, (the 6 vertical lengths) how can I calculate AB & CD?

Thanks
Originally Posted by ThePerfectHacker
I think you need more than that. How about the widths?
I agree with TPH. Once you set the distance between any two of the heights you can calculate the rest.

-Dan

4. Originally Posted by emersong
Just wondering if someone can help me with a work problem please.

From the picture, if I only know the 6 lengths of a,b,c,d,e, & f, (the 6 vertical lengths) how can I calculate AB & CD?
Hello,

the length of (AB) is calculated by:

$(AB)^2 = (f-a)^2 + (CD)^2$

The length of (AB) is related to the length of (DC). That means you can't calculate (AB) and (CD) independently. (Compare the previous posts of ThePerfectHacker and topsquark)

5. Originally Posted by ThePerfectHacker
I think you need more than that. How about the widths?
I don't know the widths, at least they would be quite difficult to measure accurately as they are in a machine with no real datum points. I can measure between AB and CD fairly easily but it would be difficult to get a datum for the distance between any two lengths.

I am only really interested in the relative angle between AB and CD, as these are 2 parts in a machine that move and the relative angles are important.

I thought that if I just measure across AB and CD at any 3 or points, I would be able to fit a straight line. (Which I guess I can do manually via Visio or AutoCAD.)

Would it help if all the lengths were at right angles to CD?

6. Originally Posted by emersong
I don't know the widths, at least they would be quite difficult to measure accurately as they are in a machine with no real datum points. I can measure between AB and CD fairly easily but it would be difficult to get a datum for the distance between any two lengths.

I am only really interested in the relative angle between AB and CD, as these are 2 parts in a machine that move and the relative angles are important.

I thought that if I just measure across AB and CD at any 3 or points, I would be able to fit a straight line. (Which I guess I can do manually via Visio or AutoCAD.)

Would it help if all the lengths were at right angles to CD?
So you only want to know the relative angle between AB and CD. I assume this has something to the with the slope of AB if CD is horizontal.

Let that angle be alpha.

Get the meauments of AD and BC, and of CD.
Slope of AB = (BC -AD) /CD
Then, tan(alpha) = slope of AB