Respuesta :

[tex]\bf the\ 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}\\ d=\textit{common difference} \end{cases}\\\\ -----------------------------\\\\ \textit{Sum of a finite arithmetic sequence}\\\\ S_n=\cfrac{n}{2}(a_1+a_n)\qquad \begin{cases} n=n^{th}\ term\\ a_1=\textit{first term}\\ \end{cases}\\\\ -----------------------------\\\\ S_{12}\impliedby \textit{means n = 12}\qquad thus \\\\\\ [/tex]

[tex]\bf S_{12}=\cfrac{12}{2}(7+a_{12})\impliedby \textit{so what's }a_{12}?\qquad well \\\\\\ a_{12}=7(12-1)(-8) [/tex]

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