把a=x/(y+z),b=y/(z+x),c=z/(x+y)
代到 a/(a+1)+b/(b+1)+c/(c+1)
得 [x/(y+z)]/[1+x/(y+z)]+[y/(z+x)]/[1+y/(z+x)]+[z/(x+y)]/[1+z/(x+y)]=[x/(y+z)]/[(x+y+z)/(y+z)]+[y/(z+x)]/[(x+y+z)/(z+x)]+[z/(x+y)]/[(x+y+z)/(x+y)]=x/(x+y+z)+y/(x+y+z)+z/(x+y+z)=(x+y+z)/(x+y+z)=1