Question

a soma dos 15 primeiros termos da P.A. (-4,1,...)

Answer 1

Calcular primeiro o ´"an"

$a_n=? \\ a_1=-4 \\ n=15 \\ r=1-(-4)=1+4=5 \\ \\ termo~~geral \\ a_n=a_1+(n-1)r \\ \\ a_n=-4+(15-1)(5) \\ a_n=-4+(14)(5) \\ a_n=-4+70 \\ a_n=66$

vamos a soma

$termo~~geral \\ \\ S_n= \frac{(a_1+a_n)n}{2} \\ \\ S_{15}= \frac{(-4+66)15}{2} \\ \\ S_{15}= \frac{(62)(15)}{2} \\ \\ S_{15}= \frac{930}{2} \\ \\ S_{15}=465$