Question

Ache a soma dos 40 primeiros termos da P.A (8,2, ...) 

Answer 1

$an=a1+(n-1)r\\ an=8+(40-1)(-6)\\ an=8+(39)(-6)\\ an=8+234\\ an=242\\ \\ Sn=(\frac { a1+an }{ 2 } )n\\ \\ Sn=(\frac { 8+242 }{ 2 } )40\\ \\ Sn=250*20\\ Sn=5000$

Answer 2

a1 = 8
an = ?
n = 40
r = 2-8 = -6

an = a1 + (n-1) . r
an = 8 + (40-1) . -6
an = 8 + 39 . (-6)
an = 8 + (-234)
an = -226

S = ( a1 + an ) n /2

S = ( 8 + (-226) . 40 /2

S = ( 8 - 226) . 40

Resposta = -4360