2x√(1-x2) ≤ x2+1-x2 来源于
{x - √(1-x2)}^2 ≥ 0
x^2 - 2x√(1-x2) + (1-x2) ≥ 0
x^2 + (1-x2) ≥ 2x√(1-x2)