由A/B=C/D=(A+C)/(B+D)得:
x/y
=(y+x+y)/(x-z+z)
=(x+2y)/x
=1+2*y/x
设x/y=m
则m=1+2/m
m*m-m-2=0
解得m=-1(舍去);m=2
即x/y=2