(1)x1+x2=0 (2)①x1^2+x2^2 ②1/x1+1/x2
k^2-4k-3=0 =(x1+x2)^2-2x1x2 =x1+x2
k^2-4k=3 x1+x2=3 x1x2=-5 x1+x2=3
k^2-4k+4=7 代入得：x1^2+x2^2 代入得：1/x1+1/x2=3
(k-2)^2=7 =3^2-2×(-5)
k-2=±√7 =9+10
k=2±√7 =19
即k1=2+√7
k2=2-√7
分析：运用韦达定理即可快速解决.