设x/a=y/b=z/c=p,则:(x+y)/(a+b)=(y+z)/(b+c)=(z+x)/(c+a)=p
原式=(x/a)(y/b)(z/c)[(a+b)/(x+y)][(b+c)/(y+z)][(c+a)/(z+x)]
=p³/p³
=1