y = log2(x^2 - 4x + 4)
= lg(x^2 - 4x + 4)/lg2
= lg[(x - 2)^2]/lg2
(x - 2)^2 > 0
x > 2 或者 x < -2
lg2 > 0
x∈(-∞,-2) lg[(x - 2)^2] 单调递减,y 单调递减
x∈(2,∞) lg[(x - 2)^2] 单调递增,y 单调递增