x1+x2=m;
x1x2=2m;
x1²+x2²=(x1+x2)²-2x1x2=m²-4m=7;
(m-2)²=11;
m=2±√11;
Δ=m²-8m≥0;
m≥8或m≤0;
∴m=2-√11;
(x1-x2)²=x1²+x2²-2x1x2=(x1+x2)²-4x1x2=m²-8m=7-4m=7-4(2-√11)=4√11-1;
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