1/x+1/(y+z)=1/2
(y+z)/[x(y+z)]+x/[x(y+z)]=1/2
(x+y+z)/[x(y+z)]=1/2
1/x=(y+z)/[2(x+y+z)]
同理可得：
1/y=(x+z)/[3(x+y+z)]
1/z=(x+y)/[4(x+y+z)]
∴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
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