Question

soma dos 15 primeiro termo da p.a (-4,1,...)

Answer 1

$\mathtt{R = A_2 - A_1} \\ \mathtt{R = 1 - (-4)} \\ \mathtt{R = 1 + 4} \\ \mathtt{R = 5}$


$\mathtt{A_n = A_1 +(n - 1)~.~R} \\ \mathtt{A_{15} = -4+(15-1)~.~5} \\ \mathtt{A_{15}=-4+ 14~.~5} \\ \mathtt{A_{15} = -4 +70} \\ \mathtt{A_{15}=66}$

$\mathtt{S_n = \dfrac{(a_1 +a_n)~.~R}{2}~~} \\ \\ \\ \mathtt{S_{15}=\dfrac{(-4+66)~.~15}{2}~~=~~\dfrac{62~.~15}{2}~~=~~\dfrac{930}{2}~~=~~465} \\ \\ \\ \boxed{\boxed{\mathtt{Resposta: 465}}}$