Originally Posted by

**Maximus**
' if the question to be answered is "How does the current change if the voltage changes," you use the derivative, not V/I.'

Completely agree!

The question is asking to find R at a particular value of I as you explained below:

'If, for example, the goal is to replace the lamp with an ordinary resistor and maintain the same power consumption, then one computes the value of that ordinary resistance with the simple ratio V/I at the current you specified.'

So why not V/I in this case?