Question: dsolve of a constant quotient (Isoperimetric problem)

I am working on an Isoperimetric problem.

I have two différential dL and dA. And I am seeking the function y for wich the two différentials are collinear.

So I want "dL/dA=constant"


I tried many expression, but I don't know how to express the "= constant"
I tried also (dL/dA)'=0 but no workable answer...


Thank's for your help !

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