Question

determine o 12° termo da PA(1, -7, -15, -23...)

Answer 1

Dada a P.A:

$\mathsf{\left(1,-7,-15,-23,...\right)}$

Determinando o valor da razão:

$\mathsf{r=a_n-a_{n-1}}\\\mathsf{r=a_2-a_1}\\\mathsf{r=-7-1}\\\mathsf{r=-8}$

Substituindo na formula geral da P.A teremos:

$\mathsf{a_n=a_k+\left(n-k\right)r}\\\mathsf{a_{12}=a_1+\left[\left(12-1\right)\left(-8\right)\right]}\\\mathsf{a_{12}=1-\left(8\cdot 11\right)}\\\mathsf{a_{20}=1-88}\\\mathsf{a_{12}=-87}\: \: \checkmark$

Answer 2

resolução!

r = a2 - a1

r = - 7 - 1

r = - 8

a12 = a1 + 11r

a12 = 1 + 11 * (-8)

a12 = 1 + (-88)

a12 = - 87