Intro Example:  Suppose in a theater the first row has 11 seats.  Every row after that has 2 more seats than the previous row.  How many seats are on the 13th row?  

To answer this, we can use the understanding of an arithmetic sequence.  An explanation for an arithmetic sequence is given below.  Once you understand the concept, you can come back and determine how many seats are on the 13th row.

Arithmetic Sequence: A sequence of numbers that have a common difference.  In the example below, the common difference (defined by the difference in a value in the sequence – the previous value in the sequence) is -50.