函数y=log2[x^2+(a-1)x+(a+1)/4]的定义域为全体实数所以x^2+(a-1)x+(a+1)/4>=0不管x取何值都要成立，判别式b^2-4ac=(a-1)^2-4*((a+1)/4)<0========>a^2-6a-3<0===>3-2根号3
