Question

Exercício sobre PA:
Numa PA, Sn=324,a1=4 e an=68. Determine n e a razão.

Answer 1

$a_{1}=4\\a_{n}=68\\S_{n}=324$

$S_{n}=(a_{1}+a_{n})*n/2\\324=(4+68)*n/2\\324=72*n/2\\324=36*n\\324/36=n\\n=9$

$a_{n}=a_{1}+(n-1)r\\68=4+(9 - 1)r\\68-4=8r\\64=8r\\64/8=r\\r=8$