解析：y=(log2 x/8)(log2 x/4)
=（log2 x-3)(log2 x-2)
=(log2 x)^2-5log2 x+6
=(log2 x-5/2)^2-1/4
令log2 x-2/5=0,则
x=4倍根号2,故 当2=