# Dirac Delta function question

I cannot get the answer as from the solution manuel.

Please tell me what am I assuming wrong.
Thanks

Dick
Homework Helper
Just showing that the value of a function is '-infinity' at a point and zero otherwise doesn't tell you it's a delta function. Apply the divergence theorem to grad(1/r) on a sphere around the origin to figure out how much 'charge' is there.

Just showing that the value of a function is '-infinity' at a point and zero otherwise doesn't tell you it's a delta function. Apply the divergence theorem to grad(1/r) on a sphere around the origin to figure out how much 'charge' is there.

I understand that and I showed in my own answer. What I don't get is where the 4$$\pi$$ come from!!
Thanks

Dick