since it seems to work with an ideal opAmp: verify the opAmp parameters and/or set initial conditions of the opAmp

Others have answered your question implicitly by suggesting assumptions. An answer to your title could be: Because Maple does not know where to locate x^2+y^2 in the complex plane. Depending on where it is located the simplified result can differ in sign.
If x and y were imaginary for example, Maple could simplify to something that does not appear equal, such as:

In your example simplify with the option symbolic works as well (but assumptions are the safer way, seeQues_Mapleprime_symbolic.mw)

Download root_in_fraction.mw

For repeated use you could think of defining a function that makes live a bit easier and avoids nested commands with many parenthesis.

I have taken you inital question as an example in an unrealted question. 

Maybe such a construct can help in the future when no "one-liner" is at hand.

In case you don’t get better material, I can at least share my thoughts and experience about frames and flanges.

Since MapleSim is very easy to start without robotics background, there is the danger that flanges and frames are mixed up by wrong intuition.

The reason is that 3D multibody components orient to each other via (coordinate) frames. During assembly, when drawing a connection between 2 components frames match together as if they were true mechanical flanges with bolts and nuts.

Another reason for misleading intuition can be that the very important RBF component (rigid body frame) looks like a structural component with flanges at the ends.

In the attached example 2 strategies to build an assembly are shown. I prepared the example to better understand what frames in MapleSims provide (today I would call the “matching frame approach” rather “rotating frame approach”. Maybe the example helps understanding frames.

tasks,visualization,highlightingMultibodyPortsAxes (paste it in the help system) might also be useful to understand frames and inspect assemblies. You will also see that connecting components in 3D is actually done via ports and that these ports have axes of a (coordinate) frame that align to each other. So frames in my interpretation are a kind of 3D port to rigidly connect and orient components.

Flanges are more 1D interfaces/ports where you connect “something” that either constrains assemblies or transfers energy.

Good luck with your team!

Assembly_approaches.msim

I could fix your worksheet (see attached).

However, I agree that your worksheet should have worked. I expect the solve command to produce the same result as the isolate command. This observation should be investigated by an expert (I have send a software change request).

Maple Worksheet - Error

Failed to load the worksheet /maplenet/convert/zadanie_z_jednostakim_-_problem-_using_isolate.mw .

Download zadanie_z_jednostakim_-_problem-_using_isolate.mw

.

Alternatively, you can take advantage of Maples ability to handle equations.

In the attached example, no assignment statements “:=” are used for parameters. Instead, general analytic solutions are derived and parameters for a particular solution are defined as equations (as you would write it by hand).

Using this workstyle, no re-execution of the whole worksheet is required when parameters are changed. This is an advantage if the worksheet becomes large or if parameter studies are done. Also, numerical artifacts or missmatch of units, as in your case, are less likely to impact calculations and results.

Maple Worksheet - Error

Failed to load the worksheet /maplenet/convert/Answer_on_-_Solving_equation_with_units_problem.mw .
 

Download Answer_on_-_Solving_equation_with_units_problem.mw