A1=2 d=2 an=80 n-? Sn-?

Question

A1=2 d=2 an=80 n-? Sn-?

Answer 1

S=(2a1+(n-1)d)/2*n
80=(2a1+dn-d)/2 *n
(2a1+dn-d)n=160
(6+2n-2)n=160
(2n+4)n=160
(n+2)n=80
n^2+2n-80=0
n=-10; n=8
n>0 => n=8

Ответ: 8