I have this solution that I don't quite understand(adsbygoogle = window.adsbygoogle || []).push({});

d(ro*pi*D^3/6)/d(t)=-kr*Cao*pi*D^2

from here I would have thought to separate the variables and integrate but the solution says

ro/2* d(D)/d(t) = -kr Cao

I guess my question is how are you able to do that? If you take the derivative of D, why is d(D)/d(t) still left behind?

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# Shrinking core model

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