x² = 2(1-m)x-m²x²-2(1-m)x+m²=0有两实数根，△=4(1-m)²-4m²=4(1-2m)≥0，m≤1/2y = x1+x2=2(1-m)m≤1/21-m≥1/2y = x1+x2 ≥ 1m=1/2时，y=x1+x2有最小值1
