Question

Uzma has a rock collection. She adds 2 new rocks each week. The arithmetic sequence
represents the number of rocks she has each week. Use the recursive arithmetic formula
to determine the number of rocks in the collection at the end of week 5.

19

18

21

17

Answer 1

Recursive rule: a_n = a_{n-1} + 2, so explicit: a_n = a_1 + (n−1)·2.
Thus a_5 = a_1 + 4·2 = a_1 + 8. If the sequence starts at 11 (11,13,15,17,19…), then a_5 = 11 + 8 = 19.

Answer: 19.