lg(x-2y)=1/2(lgx+lgy)
lgxy=2lg(x-2y)=lg(x-2y)^2
(x-2y)^2=xy
x^2-4xy+4y^2=xy
x^2-5xy+4y^2=0
(x-4y)(x-y)=0
x=4y或x-y=0
x/y=4或x/y=1
因为x-2y>0 x>2y
所以x/y=4