Coloumbs law with inverse cubic

[yusufali]

1. Homework Statement
A permanent electric dipole consisting of charges +q and -q
separated by the fixed distance s. Charge +Q is distance r from
the center of the dipole. We'll assume s << r.

a) Use the binomial approximation (1+x)^-n ~= 1-nx if
x << 1 to show that the net force exerted on the dipole by charge +Q
is F_net = 2KqQs/r³

Homework Equations

F = Kqq/r²

3. Attempted Solution

F = +kQq/(r+s/2)² - kQq/(r-s/2)²
F = -2kQq(rs)/( (r+s/2)² * (r-s/2)² )

i cannot figure out the rest from there...

 

[voko]

You have been instructed to use $(1 + x)^{-n} \approx 1 - nx$, when $x << 1$. Why are you ignoring this?