视 y, z 为常数，对 x 求偏导数，得 (D/Dx)[1/√(x²＋y²＋z²)] = (D/Dx)[(x²＋y²＋z²)^(-1/2)] = (-1/2)[(x²＋y²＋z²)^(-3/2)]*(2x)
