Question

Calcular a soma dos 20 primeiros termos da P.A. (15 , 13 , 11,...)

Answer 1

Resposta:

razão=r=13-15=-2

Sn=(a1+an)*n/2

an=a1+(n-1)*r

a20=15+(20-1)*(-2)

a20=15+(20-1)*(-2) = -23

S20=(15-23)*20/2  = -80