lg(x-y)+lg(x-2y)=lg2+lgx+lgy
lg(x-y)(x-2y)=lg2xy
x^2-2xy-xy+2y^2=2xy
x^2-5xy+2y^2=0
(x/y)^2-5(x/y)+2=0
x/y=5+√(5^2-4*2)/2=(5+√17)/2
或x/y=5-√(5^2-4*2)/2=(5-√17)/2