Verified answer

1) a1 = 3 ;

a17 = 35 ;

S17 = (a1+a17)/2 × 17 = 38/2 × 17 = 323

Ответ: S17 = 323

2) 5;10;15;20..

a1 = 5

d = 5

S20 = (2a1+d(n-1))/2 × 20 = (10 + 19×5) × 10 = 1050

Ответ : S20 = 1050