Question

已知x+y+z=a,xy+yz+xz=b,求x2+y2+z2的值

Answer 1

x²+y²+z²
=(x+y+z)²-2(xy+yz+zx)
=a²-2b

Answer 2

（x+y+z）^2
=x^2+y^2+z^2+2(xy+xz+yz)=a^2
x^2+y^2+z^2=a^2-2b