方差与标准差

def solve(returns, probabilities):
    import math
    expected = sum(r * p for r, p in zip(returns, probabilities))
    variance = sum(p * (r - expected) ** 2 for r, p in zip(returns, probabilities))
    std = math.sqrt(variance)
    return {"期望收益率": round(expected, 4), "方差": round(variance, 6), "标准差": round(std, 4)}