7. (5%，本題計算過程須計分) A disk of radius R has a uniform surface charge density σ (Fig. 5). Prove that magnitude of the electric field at a point P is $$E = 2\pi k_e \sigma \left[ 1 - \frac{x}{(R^2 + x^2)^{1/2}} \right]$$. Note that P lies along the central perpendicular axis of the disk and a distance x from the center of the disk. Your proof:__ (7)__