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RyanFehr1-Visitor
1-Visitor
September 26, 2011
Question

User error or glitch? "variable undefined" Mathcad 15

  • September 26, 2011
  • 2 replies
  • 5974 views

I've been using Mathcad for a couple years now and I think I have a pretty firm grasp on the in's and out's of the program, but I've always found little things that just seem to happen at random and seem to be a glitch or something. One such problem is when I copy and paste formulas from one sheet to another. The definitions sometimes are no longer recognized. And for the life of me, I cannot get Mathcad to recognize them again. Below is an example of this. This is an assignment I am working on for a class. The first part is a calculation of section properties of a channel. Everything from "Vx:" down is copy&paste'd from a previous assignment for calculating shear flow and shear center in channels. The formulas now no longer recognize the "xbar" definition. Can someone help me out with this problem? Thanks.

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2 replies

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RyanFehr1-VisitorAuthor
1-Visitor
September 26, 2011

Ah, I figured it out. I guess I need to call xbar as well as any other variable I use that's defined by a formula, regardless of whether the it's in symbolic form or not. I was under the impression that I only needed to call variables that were still in symbolic form. Guess I'm still learning.

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RyanFehr1-VisitorAuthor
1-Visitor
September 26, 2011

nevermind, thats just leaves it in symbolic form and useless to me.

BTW, I tried manually retyping the formula...that doesn't solve the problem.

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VladimirN24-Ruby III
24-Ruby III
September 27, 2011

Hello!

Please, attach your Mathcad worksheet.

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RyanFehr1-VisitorAuthor
1-Visitor
September 27, 2011

My appologies. I ended up starting a fresh sheet, dropping units and getting things to work out without errors. I couldn't tell you why it worked this time. Anyway, I just kept chugging through the assignment because its due tomorrow...err today. I'll attach the finished assignment if that helps explain what I was doing wrong before. Also, is there some sort of setting I can change to make Mathcad provide more accurate calculations? I've been having issues with what appears to be Mathcad rounding errors. As you can see in the shear center calculation of this sheet, my shear force in the top flange doesn't quite equal -0.5lbs as it is supposed to. Thanks for the help. Sorry for the delay.

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RichardJ19-Tanzanite
19-Tanzanite
September 27, 2011

I ended up starting a fresh sheet, dropping units and getting things to work out without errors.

It it works without units but not with units that's usually a bad sign.

Also, is there some sort of setting I can change to make Mathcad provide more accurate calculations? I've been having issues with what appears to be Mathcad rounding errors. As you can see in the shear center calculation of this sheet, my shear force in the top flange doesn't quite equal -0.5lbs as it is supposed to.

The number is accurate, given your input values. So yes, it's a roundoff problem, but the roundoff is in the input numbers, not the calculations.

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