Question

Calcule a soma dos trinta primeiros termos da p.a. (-15,-11,-7,-3,...).
A)1200
B)1250
C)1270
D)1290

Answer 1

r = -11-(-15) = -11+15 = 4

a₃₀ = -15 + (30-1)4
a₃₀ = -15 + 116
a₃₀ = 101

S₃₀ = (-15 + 101).30
                    2
S₃₀ = 86 . 15
S₃₀ = 1290 (alternativa D)