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3. a dialectic sphere with radius r has a volumetric density...

# Question: a dialectic sphere with radius r has a volumetric density...

###### Question details
A dialectic sphere with radius R has a volumetric density that is not uniform and is given by ρ=ρ0 r, where ρ0 is a positive constant. The sphere is enclosed by a spherical shell with a radius of 2R and the density of the superficial charge is σ, as shown in the figure. Obtain the value of σ so that the electric field outside of the shell (r>2R) is cero.

So the answer is the one in top of the pic but I need help to get there
Thank you