F=k(3Q)*Q/r^2=3k(Q^2)/r^2
A与B互相接触再分开后
q(a)=q(b)=[3Q+(-Q)]/2=Q
A,B之间的静电力为
F'=kq(a)*q(b)/(2r)^2=k(Q^2)/(2r)^2=(1/4)kQ/r^2=3(1/12)k(Q^2)/r^2=F/12