Question

Найдите сумму первых 25 членов если an=3n-19

Answer 1

$a (25) = 3 \times 25 - 19 = 75 - 19 = 56 \\ a(1) = 3 \times 1 - 19 = - 16 \\ s(25) = ( - 16 + 56) \div 2 \times 25 = 20 \times 25 = 500$