[tex]\bf a_n=a_1+(n-1)d\qquad
\begin{cases}
n=n^{th}\ term\\
a_1=\textit{first term}\\
d=\textit{common difference}\\
--------------\\
a_5=100\\
n=5\\
d=4
\end{cases}
\\\\\\
a_5=a_1+(5-1)4\implies 100=a_1+(5-1)4
\\\\\\
\textit{solve for }a_1\textit{ to find the first term}[/tex]
then use the common difference "d", to get the 2nd and 3rd terms
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[tex]\bf a_5=a_1+(5-1)4\implies 100=a_1+(5-1)4\implies 100=a_1+(4)4
\\\\\\
100=a_1+16\implies 100-16=a_1\\\\ 84=a_1\quad
\begin{cases}
a_1=&84\\
a_2=84+4\to &88\\
a_3=88+4\to &92
\end{cases}[/tex]
