Question

I need help with this problem. My math teacher mentioned it not exactly being a arithmetic sequence.

Answer 1

`\begin{gathered} a_1=12 \\ a_(25)=a_1+(25-1)d \\ -60=12+24d \\ d=-3 \\ a_n=12+(n-1)(-3) \\ a_n=12-3n+3 \\ a_n=15-3n \end{gathered}`
`\begin{gathered} a_n=3=15-3n \\ 3=15-3n \\ 3n=15-3 \\ 3n=12 \\ n=(12)/(3) \\ n=4 \end{gathered}`
`\begin{gathered} a_8=15-3(8) \\ a_8=-9 \end{gathered}`
`\begin{gathered} a_n=-24 \\ -24=15-3n \\ 3n=15+24 \\ 3n=39 \\ n=(39)/(3) \\ n=13 \end{gathered}`
`\begin{gathered} a_(19)=15-3(19) \\ a_(19)=15-57 \\ a_(19)=-42 \end{gathered}`