Y = log 2 (x^2+1) - log 2 x
零和负数无对数：x＞0
Y = log 2 (x^2+1) - log 2 x = log 2 {(x^2+1)/x} = log 2 {x+1/x}
x＞0,x+1/x≥2,log 2 {x+1/x} ≥lgo 2 2 = 1
值域【1,+∞）