# Homework Help: Del(1/r) = -R/r^2

1. Jan 5, 2010

### boardbox

1. The problem statement, all variables and given/known data
let R be the separation vector from (a,b,c) to (x,y,z) and r be the magnitude of R.
Show that: del(1/r) = -R/r^2

2. Relevant equations

3. The attempt at a solution
The problem is that I keep getting a 3/2 power in the denominator when I calculate the left hand side.

r = sqrt((x-a)^2 + (y-b)^2 + (z-c)^2)

1/r = ((x-a)^2 + (y-b)^2 + (z-c)^2)^-0.5

del(1/r) = -R/[(x-a)^2 + (y-b)^2 + (z-c)^2]^(3/2)

Last edited: Jan 5, 2010
2. Jan 5, 2010

3. Jan 5, 2010

### boardbox

I see, you're right. I missed the hat on R in my text. Thanks.