Question

find the 80th term of the following arithmetic sequence. 4, 12, 20, 28

Answer 1

First of all, the formula to find the n-term of an arithmetic sequence is:

$\begin{gathered} a_n=a_1+(n-1).d;\text{ where} \\ a_n\text{ term that we need find} \\ a_1\text{ first term = 4} \\ n\text{ number of term = 80} \\ d\text{ = common diference = 8} \end{gathered}$

Now, replacing with the knowing values:

$\begin{gathered} a_n=4+(80-1)\cdot8 \\ a_n=4+79\cdot8=4+632=636 \\ a_{80}=636 \end{gathered}$

Your answer is the 80th term of that arithmetic sequence is 636.