This is a question set by my maths tutor, I answered it using the divergence theorem to get an answer of 18 pi, which is correct. But I was wondering how you would be able to get the same answer by tackling the question "from first principles", so to speak.
I've attached the question and my attempt at a solution, but I'm just not sure where I'm going wrong! Any advice would be appreciated.
My question refers to the third part of this question, the one with the cylinder
An image of my attempt at a solution
P.S. This is my first post so sorry that there's no posh maths text here!
Thanks, Andrew