2^x<=256
x<=8
所以1/2<=log2(x)<=3
令a=log2(x)
1/2<=a<=3
f(x)=[log2(x)-log2(2)][log√2( √x)-log√2(2)]
=(a-1)(a-2)
=a²-3a+2
=(a-3/2)²-1/4
1/2<=a<=3
所以a=3/2,最小值=-1/4
a=3,最大值=2