△=(2k)²-4*1*(k-1/4)=0
4k²-4k+1=0
(2k-1)²=0
k=1/2
x²+2kx+k=4分之1
x²+x+1/2=1/4
x²+x+1/4=0
(x+1/2)²=0
x1=x2=-1/2;