Question

Calcule a soma dos 15 primeiros termos da p.a.(-1,-11,-21,...)

Answer 1

an= a1 + (n-1) * r
an= (-1) + (15-1) * -10
an=(-1) + 14 * -10 
an= -141

Sn= n*(a1+ an) /2
Sn= 15*(-1+ -141) /2
Sn= -2130 /2
Sn= -1065