Question

If the common difference in an arithmetic sequence is 4, and the 20th term is 36, what is the first term?

Answer 1

Given:\(\large\color{black}{ \\[0.6 em] }\)
\(\large\color{black}{ \displaystyle {\rm \color{blue}{d}}=4 }\)\(\large\color{black}{ \\[0.6 em] }\)
\(\large\color{black}{ \displaystyle \color{blue}{a_{20}}=36 }\)

Answer 2

Formula\(\large\color{black}{ \\[0.6 em] }\)
\(\Large\color{black}{ \displaystyle a_{{\color{red}{\rm n}}}=\left(a_{\color{green}{\rm 1}}\right)+{\rm \color{blue}{d}}\left({\color{red}{\rm n}}-{\color{green}{\rm 1}}\right) }\)\(\large\color{black}{ \\[0.6 em] }\)
I labelled the corresponding parts in colors.