因为
xy+yz+zx
=(1/2)[(x+y+z)^2-(x^2+y^2+z^2)]
=-a^2/2
所以
∫(xy+yz+zx)ds=∫(-a^2/2)ds=(-a^2/2)∫ds
=(-a^2/2)*(2πa)=-πa^3