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**What exactly allows a "differential relation" form of an equation?**

I understand this in a superficial way but I'd really like some more clarification. If anybody can provide a little better understanding on this subject, please feel free to post anything at all. Even a sentence or two would be helpful.

I am reading through the optics section of my Stellar Astrophysics book and I came across the following sentences:

Using the small-angle approximation, tan(

This immediately leads to the differential relation known as the

*θ*) ≈*θ*, for*θ*expressed in radians, we find*y = fθ*.

This immediately leads to the differential relation known as the

**plate scale**,*dθ/dy*,[itex]\frac{dθ}{dy}[/itex] = [itex]\frac{1}{f}[/itex].

What I don't completely understand is how and why one can simply go from the y=fθ form to the dθ/dy = 1/f form. I understand what the equation means but I don't understand the rules behind switching from one form to the other. Are there any or can I change any two variables to differential form to get a new relation? Any help or guidance at all would be appreciated.