Hi,
I am trying to solve a cubic equation, but I am unsure where I am going wrong. Please guide me. I manually calculated the values as follows:
x1= 1231.629mm, x2=121.978mm, x3=-153.60mm
The Mathcad file is attached : Mathcad Prime09
I tried two different method but not sure where I'm doing mistake
ACCEPTED SOLUTION
Accepted Solutions
After adding the guessed value of x, the result I'm getting seems to be incorrect. Please correct me if I am making a mistake; the value should be as I mentioned above in the post.
@Ravii wrote:
Thanks, now I have added the file
OK, but it stills seems that your equation is not correct (when used with units; see answer above).
I guess the unit-less value for M should be 15.625*10^3, not 10^6.
Otherwise you aren't doing anything wrong.
If the results are not what you expect, you sure have to check and correct your equation!!
The "root" function or a solve block with "find" will always just return one of the three possible solutions. Which one depends on the guess value you provide.
But in newer versions of Prime you may also evaluate a solve block symbolically (a guess would not be necessary) and this will give you all three solutions.
And of course you also could use the symbolic "solve"
Clearing x is necessary because "solve" won't solve for variables which are already defined,
You may also use units, but first you would have to correct the error in your formula. In the following picture i did it the bold way by replacing the "1" with "1 m" 😉
The red error stems from Prime refusing to display large results. The result gets that large because the symbolics does not know anything about units. It does not know that MPa=10^6 Pa or that MPa*mm^3=N*mm, etc. So it treats the various units as unknown variables and can't simplify the expressions.
But you can assign the result to a variable and evaluate this variable numerically.
As you can see we get tiny imaginary parts in some of the solutions which we can get rid by just using the real parts.
You should also check this option (recalc the sheet after doing so). Primes symbolics still can't deal with units but at least the labels for the units are chosen correctly (most of the times) so we can evaluate the result numerically without getting an error.
Maybe the last factor should read (d-x/3) instead of (1-x/3)?
The solutions still are not the exact values you expect but at least in the same magnitude:
Hi Werner,
I have a question. When I try to copy the calculation method you used into my sheet, it shows an error message (see attached image). However, when I copy the same thing into a new sheet, it works. Do you know why it is showing an error?
Also I calculated the value of x manually (Image in right), not sure why the output of Mathcad is different
The calculation file is attached.
Probably because it is undefined. Read the error message: Check the label.
You want to make sure that every xv in your expressions has the same label.
Incidentally. If you really want the three solutions to you problem, you can (should?) do as follows:
This results in a big expression, which Prime probably cannot show, but here is:
Never the less, you should be able to define your knowns:
and calculate the numerical results:
Success!
Luc
I tried the same approach, but it didn't solve the problem. I'm not sure where I'm going wrong.
Move the definitions of M, b,d,m and t BELOW the x(...):=....solve,x expression.
If you then get the same 'result', Prime has proven itself incapable (once more).
{You may want to check for certainty by trying to solve
}
Success!
Luc
Same issue 😞
Its better to solve it manually. I thought it will be easier if it done in Mathcad.
