x^2-2kx+1=k^2x^2-2kx+1-k^2=0x1+x2=2kx1x2=1-k^2x1^2+x2^2=(x1+x2)^2-2x1x2=4k^2-2(1-k^2)=4k^2-2+2k^2=6k^2-2判别式=4k^2-4(1-k^2)=4k^2-4+4k^2=8k^2-4>=0k^2>=1/2k=根号2/2x1^2+x2^2的最小值1