What Is the Electric Field Magnitude at r for Concentric Spheres?

[divr]

There are three concentric thin spheres of radii a,b, c (a>b>c). The outer surface charge densities of their surfaces are σ, -σ and σ respectively. Find the magnitude of electric field at r (distance from centre) such that a>r>b ??

My answer was,

Er = [ K σ . 4 (pi) c² / r² - K σ . 4 (pi) b² / r² ]

But the answer is given as,

4(pi) b² / r²

 

[ideasrule]

If 1/K=epsilon, I think your answer is correct.