The directional derivative of 1/r in the direction of  is

Concept:

Let f(r) be a function then directional derivative of the function f(r) is given by: 

Calculation:

Given:

f(r) = 1/r

As we know that, if f(r) is a function then directional derivative of the function f(r)is given by: 

f(r) = 1/r

∴ f'(r) = 

Here, we have to find the directional derivative of f(r) in the direction of . It will be given by: