Seikoid
Answered

which term of the arithmetic sequence is -18, given that a1=7 and a2=2?

Sagot :

Mariamikaylaid
[tex]a_n=-18 \\\\ a_1\to 7 \\ a_2\to 2 \\\\ a_2=a_1+r \\\\ 2=7+r \\\\ 2-7=r \\\\ \boxed{r=-5} \\\\\\ a_n=a_1+(n-1)*r \\\\ -18=7+(n-1)*(-5) \\\\ -18-7=(n-1)*(-5) \\\\ -25=(n-1)*(-5) \ \ \ |:(-5) \\\\ 5=n-1 \\\\ 5+1=n \\\\ \boxed{n=6} \Longrightarrow \boxed{\boxed{a_6=-18}}[/tex]
Mlcparra16id
Please note that:
[tex]a_2-a_1=a_3-a_2=a_4-a_3=...=a_n-a_{n-1}=d[/tex]
[tex]a_2-a_1=2-7=-5=d[/tex]

[tex]n= \frac{a_n-a_1}{d} +1= \frac{-18-7}{-5} +1= 6[/tex]