Series in real-life models

The idea

Steady increases make arithmetic series: a salary rising by the same amount each year. Percentage changes make geometric series: savings growing by $3\%$ a year, or a ball bouncing to $60\%$ of its height.

1. Decide: same amount each time (arithmetic) or same percentage (geometric).
2. Write $a$ and $d$ or $r$ from the situation.
3. Check which term or how many terms the question means.

Watch out Count carefully: the salary in year $5$ is the 5th term, $a + 4d$ or $ar^4$.