## Precalculus (6th Edition) Blitzer

$4,8,12,16,20,...$ Arithmetic.
When we observe the sequence carefully, it is seen that the difference between two consecutive terms is the same in the series $4,8,12,16,20,...$ Now, \begin{align} & {{a}_{1}}=4,{{a}_{2}}=8, \\ & {{a}_{3}}=12,{{a}_{4}}=16 \\ & {{a}_{5}}=20,... \\ \end{align} Then, \begin{align} & {{a}_{2}}-{{a}_{1}}=8-4 \\ & =4 \\ & {{a}_{3}}-{{a}_{2}}=12-8 \\ & =4 \end{align} \begin{align} & {{a}_{4}}-{{a}_{3}}=16-12 \\ & =4 \\ & {{a}_{5}}-{{a}_{4}}=20-16 \\ & =4 \end{align} Therefore, the common difference is 4. Hence, the sequence is Arithmetic.