xy+yz+zx=[(x+y+z)^2-(x^2+y^2+z^2)]/2
原式=(xy+xz+yz)(x+y+z)=-1/4
4.若已知x2+y2+z2= ,x+y+z= ,試求(x+y)(y+z)(z+x..-阿摩線上測驗