Question

culcular a soma dos 80 primeiros termos da P.A(6, 9, 12, 15, 18)

Answer 1

Sn = (a1+an)*n/2
an = a1+ (n-1)*r
an= 6+(80-1)*3
an= 6+ 79*3
an= 6+ 237
an= 243

S80= (6+243)*80/2
S80= 249 * 80/2
S80= 19920/2
S80= 9960

Answer 2

an=a1 + (n - 1) . r
an=6 + (80 -1) . 3
an=6 + 79 . 3
an=85 . 3
an=255


Sn=(a1 + an) . n
        2

Sn=(6 + 255) . 80
            2

Sn=261.40

Sn=10440