A positive charge Q is on the y-axis at a distance a from the origin, and another positive charge q is on the x-axis at a distance b from the origin.

Question

A) For what values(s) of b is the x-component of the force on q a minimum?
B) For what values(s) of b is the x-component of the force on q a maximum?

Answer 1

when b is zero, there is no x component

Now, the x component of force is

Fx=kqq/(a+b) * cosTheta

where cosTheta= b/(sqrt(a+b))

so, take the deriviative of Fx with respect to b, and solve.