What is an Arithmetic Sequence?

What is an Arithmetic Sequence? Give a good example.

2. Define the general term in arithmetic Sequence and refer its Key ingredients.

3. Write a formula for the general term (the nth term) of the arithmetic sequence whose second term, a2, is

4 and whose sixth term, a6, is 16.

4. Write a formula for the general term (the nth term) of the arithmetic sequence whose third term, a3, is 7 and whose eighth term, a8, is 17. 

(What formula do we use to solve the above two problems)