Added a python script for finding sum of arithmetic series (#1279)

* Added a python script for finding sum of arithmetic series

* Added some linting

* Resolved comments

* Fixed flake8 test

maths/sum_of_arithmetic_series.py

@@ -0,0 +1,22 @@
+# DarkCoder
+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
+
+
+def main():
+    print(sum_of_series(1, 1, 10))
+
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()