f(x) = log0.5(4x)·log0.5(x/8)
= { log0.5(4) + log0.5(x)} {log0.5(x)-log0.5(/8) }
= { -2 + log0.5(x)} {log0.5(x)+3 }
= {log0.5(x)+3 } {log0.5(x)-2 }
= {log0.5(x)}^2+ log0.5(x) - 6
= { log0.5(x)+1/2 }^2 - 25/4
x属于（1/2,4]
log0.5(x)属于[-2,1)
log0.5(x)+1/2属于[-3/2,3/2)
{ log0.5(x)+1/2 }^2属于【0,9/4)
{ log0.5(x)+1/2 }^2-25/4属于【-25/4,-4)
值域：【-25/4,-4)