rv_interface.py 文件源码

def variance(X, condition=None, **kwargs):
    """
    Variance of a random expression

    Expectation of (X-E(X))**2

    Examples
    ========

    >>> from sympy.stats import Die, E, Bernoulli, variance
    >>> from sympy import simplify, Symbol

    >>> X = Die('X', 6)
    >>> p = Symbol('p')
    >>> B = Bernoulli('B', p, 1, 0)

    >>> variance(2*X)
    35/3

    >>> simplify(variance(B))
    p*(-p + 1)
    """
    return cmoment(X, 2, condition, **kwargs)