解：1/x+1/(y+z)=1/2, (x+y+z)/[x(y+z)]=1/2即 1/x =(y+z)/[2(x+y+z)]同样可得：1/y=(x+z)/[3(x+y+z)]1/z=(x+y)/[4(x+y+z)]所以：2/x+3/y+4/z =(y+z)/(x+y+z)+(x+z)/(x+y+z)+(x+y)/(x+y+z)=2(x+y+z)/(x+y+z)=2
