Why dont integrate all electric fields in a sphere?

[axcelenator]

If volumetric charge distribution has spherical symmetry
I want to find the trapped charge a in certain radius
Why did not need to do an integral from 0 to R to all the electric fields inside the sphere but take only the external field(how gauss law says)?

The Electric fiels is: E=3AB*r*e^(-1.5Br^2)

 

[Simon Bridge]

You could do that, it's just that the result for the field inside the radius is zero.