因为f(x)=log2[ax^2+(a-1)x+1/4]的值域为R
所以ax^2+(a-1)x+1/4恒大于0
等价于a＞0且Δ＜0,即
(a-1)^2-a＜0
即a^2-3a+1＜0
解得(3-√5)/2＜a＜(3+√5)/2