Answer:
Explanation:
The two charges are q and Q - q. Let the distance between them is r
Use the formula for coulomb's law for the force between the two charges

So, the force between the charges q and Q - q is given by

For maxima and minima, differentiate the force with respect to q.

For maxima and minima, the value of dF/dq = 0
So, we get
q = Q /2
Now 
the double derivate is negative, so the force is maxima when q = Q / 2 .