Answer
$a_1=-1
,a_2=-5
,a_3=-9
,a_4=-13$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To find the first four terms of the given sequence
\begin{array}{l}\require{cancel}
a_1=-1
\\\\
a_n=a_{n-1}-4,
\text{ if }n\gt1
,\end{array}
use the given value for $a_1.$ Then use substitution to get the next $3$ terms.
$\bf{\text{Solution Details:}}$
Using the value of $a_1$ and $n=2,$ the second term is
\begin{array}{l}\require{cancel}
a_2=a_{2-1}-4
\\\\
a_2=a_{1}-4
\\\\
a_2=-1-4
\\\\
a_2=-5
.\end{array}
Using the value of $a_2$ and $n=3,$ the third term is
\begin{array}{l}\require{cancel}
a_3=a_{3-1}-4
\\\\
a_3=a_{2}-4
\\\\
a_3=-5-4
\\\\
a_3=-9
.\end{array}
Using the value of $a_3$ and $n=4,$ the fourth term is
\begin{array}{l}\require{cancel}
a_4=a_{4-1}-4
\\\\
a_4=a_{3}-4
\\\\
a_4=-9-4
\\\\
a_4=-13
.\end{array}
Hence, the first four terms are $
a_1=-1
,a_2=-5
,a_3=-9
,a_4=-13
.$
