由(1/x)+1/(y+z)=1/2得(x+y+z)/x(y+z)=1/2，2/x=(y+z)/(x+y+z)同理，3/y=(x+z)/(x+y+z)，4/z=(x+y)/(x+y+z)所以2/x+y/3+z/4=(y+z+x+z+x+y)/(x+y+z)=2
