Add implementation for nth_term in SML.

.gitignore

@@ -1 +1,5 @@
 /bin/
+
+# OS X
+*.DS_Store
+

algebra/arithmetic_progression/sml/arithmetic_progression.sml

@@ -0,0 +1,33 @@
+(*
+ * Brief: Calculates the nth term in an arithmetic sequence.
+ *
+ * Note, we start the sequence from n = 0 (i.e. the first number in the
+ * sequence is when n = 0).
+ *
+ * Requires: n >= 0
+ *
+ * Param a: starting value
+ * Param d: difference between each term
+ * Param n: number in the sequence we want to get
+ *
+ * Return: The nth number in an arithmetic sequence.
+ *)
+fun nth_term (a: int) (d: int) (n: int): int =
+  if n = 0
+  then a
+  else d + nth_term a d (n - 1)
+
+(*
+ * Brief: Calculates the sum of the first n numbers in an arithemetic sequence.
+ *
+ * Requires: n >= 0
+ *
+ * Param a: starting value
+ * Param d: difference between each term
+ * Param n: number of terms we want to add
+ *
+ * Return: The sum of the first n numbers in an arithmetic sequence.
+ *)
+fun sum_of_first_n (a: int) (d: int) (n: int): int =
+  (n * (2 * a + (n - 1) * d)) div 2
+