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Thank you very march. It works.
Another way Clear[x, y, z]; HoldForm[(x + y)/z] /. {x -> 2, y -> 3, z -> 5} To calculate the expression use ReleaseHold[%]
Build in functions work faster than program code.
You try to DSolve[] nonlinear differential equation. The probability to obtain analytical solution is very small. Try to solve it numerical with NDSolve[].
You can't 3Dplot your list because each point has 4 coordinates. Something is wrong with your question.
I think that you can write directly ![enter image description here][1] [1]:
I needed to make the first order Taylor series with the function Series. I received a second order term in the result. Mathematica version on Mac. I think that something is wrong with the function Series[]. I take a simple function as...