Given:
[tex]\begin{gathered} t_n=t_{n-1}+7 \\ First\text{ term, }t_1=-5 \end{gathered}[/tex]
Hence we can write,
[tex]d=t_n-t_{n-1}=7[/tex]
Here, d is the common difference.
Now, the n th term formula can be written as,
[tex]t_n=t_1+(n-1)d[/tex]
So, the term t4 can be found as,
[tex]\begin{gathered} t_4=-5+(4-1)7 \\ =-5+3\times7 \\ =-5+21 \\ =16 \end{gathered}[/tex]
Now, the term t20 can be found as,
[tex]\begin{gathered} t_{20}=-5+(20-1)7 \\ =128 \end{gathered}[/tex]
So, the answers can be written as,
[tex]\begin{gathered} t_4=(-5)+(3)(7) \\ t_4=16 \end{gathered}[/tex][tex]\begin{gathered} t_{20}=(-5)+(19)(7) \\ t_{20}=128 \end{gathered}[/tex]
