求函数y=log2(x^2+1/4)-log2x（2为底）值域
问题描述：
求函数y=log2(x^2+1/4)-log2x（2为底）值域
答
log2(x^2+1/4)-log2x=log2[(x^2+1/4)/x]
f(x)=[(x^2+1/4)/x]
=x+1(4x) x>0
f(x)'=1-1/[4(x^2)]
1/(4x^2)>1时,f(x)'