y=log2 x+logx 2x
=log2 x+log2 2x/log2 x
=log2 x+(log2 2+log2 x)/log2 x
=log2 x+1/log2 x+1
这时用基本不等式
|log2 x+1/log2 x|≥2
所以log2 x+1/log2 x≥2 或≤-2