Find the maximum value of the interval of the function f (x) = log2 (4x) * log2 (8x), X ∈ [1 / 8,4]

Find the maximum value of the interval of the function f (x) = log2 (4x) * log2 (8x), X ∈ [1 / 8,4]

F (x) = (log24 + log2x) (log28 + log2x) = (2 + log2x) (3 + log2x) = (log2x) ^ 2 + 5log2x + 6, let t = log2x, X belong to [1 / 8,4], then t belongs to [- 3,2], so g (t) = T ^ 2 + 5T + 6 = (T + 5 / 2) ^ 2 + 6-25 / 4 = (T + 5 / 2) ^ 2-1 / 4, the symmetry axis is t = - 5 / 2, so the minimum value is g (- 5 / 2) = - 1 / 4, the maximum value is