import math


def bl_area(sides: list) -> float:
    """
    This function calculate area of triangle.
    :param sides: List of 3 sides of triangle
    :return: Area of triangle
    """
    half_perimeter = (1 / 2) * (sides[0] + sides[1] + sides[2])
    return math.sqrt(
        half_perimeter * (half_perimeter - sides[0]) * (half_perimeter - sides[1]) * (half_perimeter - sides[2]))