PA:
[tex]a_{17}=-40\\ r=-3[/tex]
Termo geral de uma PA:
[tex]\boxed{a_n=a_k+(n-k)r}[/tex]
[tex]a_{17}=a_1+16r\ \therefore\ -40=a_1+16(-3)\ \therefore\[/tex]
[tex]-40=a_1-48\ \therefore\ a_1=-40+48\ \therefore\ \boxed{a_1=8}[/tex]
