How Can I Resolve Conflicting Methods for Transformer Resistance Calculations?

[david13579]

Homework Statement

This is the problem: http://i.imgur.com/MrRwi.png
I'm not exactly looking for a solution to the problem, I am looking for a solution to the conflict created by different methods I've seen of doing it.

Cramster's method gives an answer of 87.5 and a solution manual i have also says 87.5, the answer on the back of my textbook also says 87.5 but when webassign gives the same problem with different numbers neither method works. My friend contacted our professor and he told her a method that does work with webbassign but that method does not get 87.5 for the numbers in the book problem so either our book, cramster and my solution manual are all wrong at once or my professor and webbassign are wrong.

One thing to keep in mind is that the solution manual gives a method for finding Vs but for it you'd need the answer (since that method is what eventually leads to the method for the answer anyway). I tried one of the webassign problems, inputting the answer to find Vs and i find a different vs than what is given, then using this new Vs gives the answer webassign gives if you follow the cramster/solution manual method.

The most intuitive way of doing it for me is how Cramster does it and that is how I would have done it. The way my professor says is to fist find Vr with the formula Vr= Sqrt (Vs^2 - V1^2) and then find Rs with = Vr/I1.

 

[jbsteele]

Then you'd use the formula Rs= (V2^2 - Vs^2) / (2*(Vs-V2)) to get Vs. This method does work for webassign and with the numbers given in the problem, it gives an answer of 90.4So basically my question is, how can I reconcile the difference between the two methods? Is one of them wrong, if so which one and why?Homework Equations Vr = sqrt (Vs^2 - V1^2) Rs = Vr/I1Rs = (V2^2 - Vs^2) / (2*(Vs-V2))The Attempt at a SolutionI think the first method is correct and that Webassign is wrong. To reconcile the difference, I would first check the formula used by webassign to make sure it is correct. If the formula is correct, then I would try to find any discrepancies in the calculation process, such as incorrect values for the inputs.