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9-Granite

## how to do the goal seek in mathcad prime 3.1

Hi all,

Please suggest, how to do goas seek  in the below attached calculation - mathcad prime 3.1

8 REPLIES 8
24-Ruby V
(To:ranto)

You have 8 equations but you just want to solve for one single variable? This may work, but mostly doesn't as the equations are likely to be contradictory/conflicting,

But ... I see that you have in total 12 variables in your 8 equations which had not been assigned any value.

These are Sfmax, W, M.o, L, S.R, S.t, S.TO, S.H, thetaf.max, V, g.o, and h.o.

I guess that it was a typo that you wrote "e" (Euler number) instead of "E" in the fourth equation!?
I also guess that V and W are meant to be variables and not the units volt and watt.

So actually you are about to solve a system with 12 unknowns but only have 8 equations!!
This may work, but usually there will be an infinite number of possible solutions and Prime will chose just one of them which might not be the one you would prefer.

To solve that kind of system you have to assign a guess value for every 12 unknowns and you also have to solve for all 12, not just for the one you are interested in only!

In any case you should correct that and if it doesn't work you should attach your worksheet, not just the pdf.

P.S.: I am not sure if your equation/condition using the "max" function will work as desired in a solve block.

9-Granite
(To:Werner_E)

23-Emerald III
(To:ranto)

Mo is undefined. 'deg' is undefined.

Please first check that ALL errors regarding undefined variables are corrected, before posting.

Success!
Luc

9-Granite
(To:LucMeekes)

Dear,

Regards,

Robert

23-Emerald III
(To:ranto)

If you:

1. Add ALL variables you want to find to the find statement, like

2. Change

to

You will find:

To be A numeric solution, I leave it up to you to decide if it's THE solution.

Success!

Luc

24-Ruby V
(To:LucMeekes)

I removed the angle theta from the solve block as its not used in the calculation. After you derived a suitable value for Sf.max you can easily calculate this angle using the derived value.

You can add constraints in the solve block.

For example I demanded Sf.max to be greater than 3 Pa (and not around zero as in the solution otherwise suggested).

As you can see in the screenshot, Prime assigns exactly that value to Sf.max and adopts the other values accordingly.

But when I change this constraints to Sf.max > 4 Pa (or higher), no solution is found. So if you expect a value around 700 MPa as your guess suggests, I would suggest that you check your formulas.

We finally arrived at seven equations in seven unknowns, but the fact, that my constraint resulted in a quite different solution and that changing the 3 Pa to 2 Pa and 1 Pa also yield different solutions, may suggest that your seven equations are not independent - a further reason to double check the equations.

9-Granite
(To:Werner_E)

Thank you

and i will recheck with exccell

9-Granite
(To:ranto)

Dear,

Dear ,

Without goal seek I had completed the calculation (enter by manual for the iteration variable which highlighted in yellow up to reach the max stress ratio at 1.0 or 0.99)

See my calculation page
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