log3x+log3y=log3(xy)
2log3(x-2y)=log3(x-2y)^2
所以 xy=(x-2y)^2,化简后得到方程
x^2-5xy+4y^2=0,解得x=y或x=4y
因为,要满足x＞0,y＞0,x-2y＞0,所以x=4y
log2(x/y)=log2（4）=2