当然是通过计算得到的结果
r=√(x²+y²+z²),即x/r=x/√(x²+y²+z²)
那么∂(x/r)/∂x
=[√(x²+y²+z²) -x *x/√(x²+y²+z²)] /(x²+y²+z²)
=(y²+z²) /(x²+y²+z²)^(3/2)
同理∂(y/r)/∂x=(x²+z²) /(x²+y²+z²)^(3/2)
∂(z/r)/∂x=(x²+y²) /(x²+y²+z²)^(3/2)
于是三者相加即2(x²+y²+z²)/(x²+y²+z²)^(3/2)
=2/√(x²+y²+z²)=2/r
不要看到式子复杂了就不肯计算