The electric field along the axis of a uniformly charged disk of radius R and total charge Q was calculated in Example 23.9. Show that the electric field at distances x that are large compared with R approaches that of a point charge Q = aπR2. (Suggestion: First show that x/(x2 + R2)1/2 = (1 + R2/x2)-1/2 and use the binomial expansion (1 + ∂) n ≈ 1 + n∂ when ∂ << 1.)