x^2+2ax+a-4=0
判别式Δ=(-2a)^2-4(a-4)
=4a^2-4a+16
=4(a^2-a+1/4)+15
=4(a-1/2)^2+15
因为(a-1/2)^2≥0,所以4(a-1/2)62+15≥15>0
所以总有两个不等实数根