y=[f(x)]^2+f(x^2)
=4+4log2x+(log2x)^2+2+log2x^2
=(log2x)^2+6log2x+6
设t=log2x,由16≥x≥1得4≥t≥0
y=(t+3)^2-3
当t=4,即x=16时
y有最大值46