y=x^2-kx+k-1
=(x-k/2)^2-k^2/4+k-1
(1)当X=-1时函数有最小值
k/2=-1
k=-2
y=x^2+2x-3=(x+1)^2-4>=4
(2)函数最小值为-1
-k^2/4+k-1=-1
k=0或k=4