（k²+1）x²-2kx+（k²+4）=0
判别式△=4k2-4(k2+1)(k2+4)
=4k2-4k4-16k2-4k2-16
=-4(k4+4k2+4)
=-4(k2+2)2
因为k2>=0,则k2+2>0
(k2+2)2>0
则△=-4(k2+2)2