Where does the symbol r come from in mathematics?

[requied]

Summary:: I have a question that I don't understand but you may know where r was come from.

I mentioned about my question on the addition.

 

[kuruman]

Lower case $r$ is radial distance from the axis. Upper case $R$ is resistance. Two different quantities. Besides, you can't have $dR$ on both sides of the equation you are questioning.

 

[BvU]

Look at the definition of resistivity $\rho$

 

[BvU]

$R = {\rho\;l\over A}$ is for a flat sheet with thickness $l$.
Here $dR = {\rho \, dr \over A}$ is for a tube with radius $r$ and thickness $dr$. Its area $A$ is $2\pi Lr$

 

[requied]

Okay I get it now. Thanks both of you for attention :)