x^2+2x+1-k^2=0
x1+x2=-2
x1x2=1-k^2
(x1-x2)^2=(x1+x2)^2-4x1x2=4-4(1-k^2)=4k^2
x1-x2=±2k
x1^2-x2^2=(x1+x2)(x1-x2)=-2*(±2k)=±4k=2
k=±1/2