利用a/b+c/d=(a+c)/(b+d)处理,
x/(x+1)=y/(y+2)=z/(z+3)=(x+y+z)/(x+y+z+6)
(x+y+z)/(x+y+z+6)=(x+y+z)/3
设x+y+z=t
t/(t+6)=t/3
t^2+3t=0
t=0或t=-3
即x+y+z的值为0或-3