= Equal-charge balance around a grounded sphere
{title2=$1/(4x^2)=x[(x^2-1)^{-2}+(x^2+1)^{-2}]$}
Place equal positive <point charges> at distances $d$ on opposite sides of a grounded conducting sphere of radius $a$, and set $x=d/a>1$. Each physical charge has an image $-qa/d$ at distance $a^2/d$ from the centre on its own side. The repulsion from the other physical charge balances attraction to both images precisely when the displayed equation holds. At large $x$, the two sides behave as $1/(4x^2)$ and $2/x^3$, explaining the approximate root $x\simeq8$. A constant nonzero sphere potential adds a central image and changes the force balance.
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