Formula for Series
Using R = rL/A , we see that we have to go over both lengths, so the lengths should add. The distances are in the numerator, and so the values should add.
This is just like in R = V/I (from V = IR) where the V’s add and are in the numerator!
Note: this is the opposite of capacitors when connected in series! Recall that
C = Q/V, where V is in the denominator!