we know that a₁ = 1, and aₙ = aₙ₋₁ + 2, is another way of saying, we add 2 to get the next term, namely, 2 is the common difference.
[tex]\bf n^{th}\textit{ term of an arithmetic sequence}
\\\\
a_n=a_1+(n-1)d\qquad
\begin{cases}
n=n^{th}\ term\\
a_1=\textit{first term's value}\\
d=\textit{common difference}\\[-0.5em]
\hrulefill\\
a_1=1\\
d=2\\
n=7
\end{cases}
\\\\\\
a_7=1+(7-1)2\implies a_7=1+12\implies a_7=13[/tex]
