def sum_of_series(first_term, common_diff, num_of_terms):
    """
    Find the sum of n terms in an arithmetic progression.

    >>> sum_of_series(1, 1, 10)
    55.0
    >>> sum_of_series(1, 10, 100)
    49600.0
    """
    sum = ((num_of_terms/2)*(2*first_term+(num_of_terms-1)*common_diff))
    # formula for sum of series
    return sum