Question

Calcule a soma dos 25 termos iniciais da P.A. (1, 7, 13, ...)

a) 1825
b) 1800
c) 145
d)155
e) 3650​

Answer 1

Resposta:

a) 1825

Explicação passo a passo:

$a_{1} =1\\\\a_{2} =7\\\\r=a_{2} -a_{1} =7-1=6\\\\n=25\\\\\\a_{n} =a_{1}+(n-1)\times r \\\\\\a_{25}=1+24\times6=1+144=145\\\\\\\\S_{n} =\frac{(a_{1} +a_{n})\times n}{2} \\\\\\S_{25} =\frac{(1 +145)\times 25}{2} \\\\S_{25} =\frac{146\times 25}{2}\\\\S_{25} =73\times 25\\\\S_{25} =1825$