Question

Determine o décimo quinto da P.A ( 1, 8, 15)

Answer 1

$a_1 = 1\\ r = 7\\ n = 15\\ a_n = ?\\ \\ a_n = a_1 + (n-1).r\\ a_{15} = 1 + (15 - 1).7\\ a_{15} = 1 + 14.7\\ a_{15} = 1 + 98\\ {a_{15} = 99}$

Resposta: 99

Answer 2

$r=a_2-a_1\\ \\ r=8-1\\ \\ r=7$

$a_{n}=a_1+(n-1)r\\ \\ a_{15}=1+(15-1)*7$

$a_{15}=1+(14*7)\\ \\ a_{15}=1+98\\ \\ \boxed{\boxed{a_{15}=99}}$

Espero ter ajudado (:
Bons estudos!