Question

if x+y+z=0 and x^2 +y^2+z^2=40, then find xy+ yz+ zx

Answer 1

Here is how to get the value of xy+yz+zx

Answer 2

(x+y+z)²=x²+y²+z²+2(xy+yz+xz)
  (0)²      =40+2(xy+yz+xz)
   -40     =2(xy+yz+xz)
 -40/2    =xy+yz+xz
xy+yz+xz=-20