Respostas

2013-08-22T22:24:27-03:00
Primeiro, calculamos a razão:

r=7-3=4

Agora, calcularemos o 25º termo:

a_n=a_1+(n-1)r\\\\
a_{25}=3+(25-1)4\\\\
a_{25}=3+24\cdot4\\\\
a_{25}=3+96\\\\
a_{25}=99

Agora podemos calcular a soma dos termos:

S_n=\dfrac{(a_1+a_n)n}{2}\\\\
S_{25}=\dfrac{(3+99)25}{2}\\\\
S_{25}=\dfrac{102\cdot25}{2}\\\\
S_{25}=51\cdot25\\\\
\boxed{S_{25}=1275}
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