For every point $A$ outside a sphere with radius $a$, there's a field $$F= \frac{K}{r^4d^2} $$ where $r$ is distance between point $A$ and the center of the sphere, and $d$ is distance between point $A$ and any point in space. (note that $r > a$)
What is the total F (sum of the fields) at point $P$ in space at distance $\tau$ from center of the sphere?