[{r^2 + (δ/2)^2 - 2r(δ/2)cosθ}^-1/2 - {r^2 + (δ/2)^2 + 2r(δ/2)cosθ}^-1/2]
=(1/r)[{1 + (δ/2r)^2 - 2(δ/2r)cosθ}^-1/2 - {1 + (δ/2r)^2 + 2(δ/2r)cosθ}^-1/2]
≒(1/r){1+(δ/2r)cosθ}-(1/r){1-(δ/2r)cosθ}
=(δ/r^2)cosθ