Δ=4k²-4(1-k²)=8k²-4≥0;
k²≥1/2;
∴k≥√2/2或k≤-√2/2;
x1+x2=2k;
x1x2=1-k²;
x1²+x2²=(x1+x2)²-2x1x2
=4k²-2(1-k²)
=6k²-2;
∴k=±√2/2时；最小值=6×(1/2)-2=3-2=1;
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