Question

Quantos termos tem a PA (250,248,...., 16).

Answer 1

Fazemos com a fórmula geral da PA:

An = a1 + (n-1).R
16 = 250 + (n-1)(-2)
16 - 250 = -2n+2
-234 = -2n + 2
-234 - 2 = -2n (-1)
236 = 2n
236/2 = n
118 = n

Existem 118 termos na PA.
(Para descobrir a razão peguei o a2 - o a1

Answer 2

$PA (250,248,...., 16)\\\\ a_{1} =250\\ a_{n} =16\\ r=-2\\n=?\\\\ a_{n} = a_{1} +(n-1)*r\\\\ 16= 250+(n-1)*(-2)\to \\\\ 16= 250-2n+2\to \\\\ 16-250-2=-2n\to \\\\ -236=-2n\to \\\\ \frac{-236}{-2} =n\to \\\\ n=118$