Question in the Attachment. Hello, maybe someone can help me in answering.
was: Frage im Anhang Hallo, vielleicht kann mir Jemand bei der Beantwortung helfen.
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It is unlikely you could find a symbolic solution with Mathematica. A numerical solution from iterating is probably feasible.
"rk ist dieUnbekannte" ~ rk is the unknown
Google Translate did not know steitig and diffbar.
stetig (instead of the typo steitig) means continuous and diffbar is short for "differenzierbar", i.e. differentiable.
PS: the question is not really Mathematica/Wolfram Language/Wolfram Alpha related.