Question

Qual a soma dos 15 primeiros termos da P.A (-45, -41, -37,. .)?

Answer 1

Sn=(a1+an)x N/2

Sn=( -675 + [-555])/2

Sn= -1230/2

Sn= -615

Answer 2

(-45, -41,-37,...)

r = - 37 - (-41)
r = - 37 + 41
r = 4
A₁ = - 45
n = 15
A₁₅= ?

An = A₁ + (n -1) . r
A₁₅= -45 + (15 - 1) . 4
A₁₅ = -45 + 14 . 4
A₁₅ = -45 + 56
A₁₅ = 11

S₁₅ = (A₁₅ + A₁) . n / 2
S₁₅ = (11 + (-45) . 15 / 2
S₁₅ = - 34 . 15 / 2
S₁₅ = - 510 / 2
S₁₅ = - 255