import math


def op_task(sum_x, sum_y, sum_a):
    """
    Return the sum_a divided by the root square of sum_x * sum_y

    :param sum_x: Sum of all X elements
    :param sum_y: Sum of all Y elements
    :param sum_a: Sum of all elements
    :return: the result of sum_a divided by the root square of sum_x * sum_y
    """

    return sum_a / float(math.sqrt(sum_x * sum_y))