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 http://latex.codecogs.com/png.latex?...C%5E%7Bn-2%7Dr

b) show that http://latex.codecogs.com/png.latex?...r.a%29%20=%20a if http://latex.codecogs.com/png.latex?%20a is a constant vector

c) find http://latex.codecogs.com/png.latex?%5Cphi such that http://latex.codecogs.com/png.latex?...%7C%5E%7B4%7Dr