a=log2(x)
1/2<=x<=4
所以-1<=a<=2
y=[log2(2)-log2(x)][log2(4)+log2(x)]
=(1-a)(2+a)
=-a²-a+2
=-(a+1/2)²+9/4
-1<=a<=2
a=-1/2,最大=9/4
a=2,最小=-4
值域[-9/4,4]