已知x+y=3,xy=-2,
则,x^2+y^2
=(x+y)^2-2xy
=3^2-2*(-2)
=9+4
=13
那么.
1/3x2+1/3y2=
=1/3*（x^2+y^2)
=1/3*13
=13/3
=4又3分之1