1≤x≤8
t=log2(x),0≤t≤3
函数f（x）=（log2 x/2)(log2 4/x)=(t-1)(2-t)=-t^2+3t-2=-(t-3/2)^2+1/4
t=3/2, x=2^(3/2,f max=1/4
t=3,x=8, f min=-2