log2(2x+8)≥3
2x+8≥8
x≥0
(1/3)^x≥1
y=(1/9)^x-(1/3)^(x+5)
=[(1/3)^x]^2-(1/3)^x(1/729)
=[(1/3)^x-1/1458]^2-1/2125764
y单调递增,当(1/3)^x=1时,有ymin=728/729
函数的值域为[728/729,+∞)