Question

Найдите сумму 27 первых членов арифметической прогрессии​! Дам еще 20 баллов Потом

Answer 1

$1)a1 = - 13 \ a2 = 2 \ d = 2 - ( - 13) = 15 \ a27 = a1 + 26d = - 13 + 26 times 15 = 377 \ s27 = frac{a1 + a27}{2} times 27 = frac{ - 13 + 377}{2} times 27 = 4914$

$2)a1 = 6.2 \ d = - 2 \ sn = frac{2 times a1 + (n - 1)d}{2} times n \ s27= frac{2 times 6.2+ 26 times ( - 2)}{2} times 27 = \ = frac{12.4 - 52}{2} times 27 = - 534.6$

$3)xfrac{}{n} = 3n - 1 \ x1 = 3 times 1 - 1 = 2 \ x27 = 3 times 27 - 1 = 80 \ s27 = frac{2 + 80}{2} times 27 = 1107$