Math Help - vector analysis, grad,div

Can anyone help me with this problem expecially c) . I believe you use a) to solve it but I don't know how.

For r = (x,y,z)

a) show that $\bigtriangledown |r|^{n}=n|r|^{n-2}r$
b) show that $\triangledown (r.a) = a$ if $a$ is a constant vector
c) find $\phi$ such that $\triangledown \phi = |r|^{4}r$

2. Choose

phi ( r ) = ( 1 / 6 ) | r |^6

3. sorry how did you come to this?

4. Originally Posted by riptorn70
sorry how did you come to this?

n - 2 = 4 so, n = 6 .

5. Thank you, Fernando Revilla.

6. Originally Posted by riptorn70
Thank you, Fernando Revilla.
You are welcome!