Question

Calcule a soma dos quinze primeiros termos da P.A. (-45 -41,-37, -33, ...)

Answer 1

a1 = - 45
a2 = - 41
r = -41 - ( - 45 )  = -41 + 45 = +4 ***
n = 15
a1 +14r = -45 + 14 ( 4 )  = -45 + 56  = 11 ***

S15 = ( -45 + 11).7,5
S15 = -34 . 7.5
S15 = -255 ***

Answer 2

Sn=(an+A1).n/2
An=A1+(n-1).r
A15=-45+14.4
A15=-45+56
A15=9
S15=(-45+9).15/2
S15=(-36).15/2
S15=-540/2
S15=-270