Question

calcule a soma dos 15 primiros termos da PA finita (-20-17-14)

Answer 1

razao =a2-a1

r= -10 -(-14)
r= -10+14
r= 4

Agora achar o termo A15


an=a1+(n-1)*r
a15= -14 +(15-1)*4
a15= -14 +14 *4
a15= -14+56
a15= 42

Soma dos termos


sn=(a1+an)*n/2
sn=(-14 42)15/2
sn=28*7,5
sn=210

espero ter ajudado

Answer 2

$\textup{Boa noite!}$

$\mathbf{R}\Rightarrow\textit{a2-a1}\Rightarrow\textit{-17-(-20)}\Rightarrow\textit{-17+20} = \textit{3}$
$\textit{N} = \textit{15}$
$\textit{A1} = -20$
______________________________________________________________
$\textit{An} = \textit{A1+(n-1).r}$
$\textit{A15} = \textit{-20+(15-1).3}$
$\textit{A15} = \textit{-20+14.3}$
$\textit{A15} = \textit{-20+42}$
$\mathbf{A15} = \mathbf{22}$

$\textit{Sn} = \frac{\mathrm{(A1+An).n} }{\mathrm{2} }$
$\textit{S15} = \frac{\mathrm{(-20+22).15} }{\mathrm{2} }$
$\textit{S15} = \frac{\mathrm{2.15} }{\mathrm{2} }$
$\textit{S15} = \frac{\mathrm{30} }{\mathrm{2} }$
$\mathbf{S15} = \mathbf{15}$

$\textup{Att;Guilherme Lima}$