Help - Problem with a simple Mathcad solution

[rctoliveira]

Hello PTC community

I am in desperate need of a hand to solve an apparently simple thermodynamics problem.

I was able to get Mathcad to run my equations, but I am guessing I made a notation-related mistake somewhere in the file, and now I just can't figure out where! I've been going through all of it for hours, but with no success. I would be most glad if one could help me out here.

The thing is:

- The answer for Part A is correct. Part B and Part C aren't (in spite of being plausible), I checked it in the book. The correct answers for these questions are

Part B:

X.1 = 0.035

x.2 = 0.19

x.3 = 0.775

P = 69.129

Part C:

x.1 = 0.109

x.2 = 0.345

x.3 = 0.546

y.1 = 0.391

y.2 = 0.426

y.3 = 0.183

v = 0.677

Thanks for the help!

[Werner_E]

In B you are mixing vector x, which has old values with variables x.1 to x.3 (literal index) with new values. You have to use one of the two notations throughout.

Keep in mind, that x.1 has absolutely nothing to do with the 3-element vector x! So its the best to use the vector notation throughout in your sheet (x[1 instead of x.1, etc).

Likewise in C - you are searching (and Mathcad modifies) the variables x.1, etc., but you still use the static vector x.

I guess the problem is that you think that x.1 references the first element of vector x as does x[1. But x.1 is a completetly different variable like x1 or x_one. It has nothing to do wit the vector x!

[rctoliveira]

In part B I tried switching the notation from x.1 to x[1 and so on, but as soon as I get to the find function and alter the variables to be in compatible notation, the software crashes. Could you check if the same happens there? If so, what could I do?

[Werner_E · Accepted solution]

Could you check if the same happens there?

No, I experience no problems replacing variables with literal indices with the appropriate vector elements and I get the correct result. See attached

[rctoliveira]

Thanks a lot, Werner, that really saved me!