Z（X+Y+Z）=5Z减去XY+YZ+ZX=3得XY=Z^2-5Z+3 (Z^2=Z*Z)而X+Y=5-Z根据[（X+Y）/2]^>=XY 柯西不等式化为关与Z的不等式3Z^2-10Z-13<=0得 -1<=Z<=13/3
