用高斯定理 ∫ E·dS = q / ε
建坐标,平板中心处x=0
在内部做一个柱面,E ΔS + E ΔS = ρ*2*x*ΔS/ ε,E = ρ*x / ε
在外部做一个柱面,E ΔS + E ΔS = ρ*b*ΔS/ ε,E = ρ*b / (2ε)