kx^2+2x-1=0有实数根
case 1: k=0
kx^2+2x-1=0
2x-1=0
x=1/2 : 有实数根
case 2 : k≠0
=>
△≥ 0
4+4k≥ 0
k≥ -1
ie
case 1 or case 2
k=0 or k≥ -1
=>k≥ -1
图
red
Δ=4-4k(-1)=4(1+k)≥0k≥-1
