The V-i graph for a conductor makes an angle $\mathrm{\theta }$ with V-axis. Here V denotes the voltage and $i$ denotes current. The resistance of conductor is given by

At an instant approach the student will choose tan θ will be the right answer. But it is to be seen here the curve makes the angle θ with the V-axis. So it makes an angle (90 – θ) with the $i$ - axis.

So resistance = slope = tan (90 – θ) = cot θ.