Вычеслите сумму первых семнадцати членов арифметической прогрессии an =9-4n

Сумма = - 459

a_n = 9 - 4n
n = 17

a_1 = 9 - 4 * 1 = 9 - 4 = 5
a_17 = 9 - 4 * 17 = 9 - 68 = -59

s_17 = (n/2) * (a_1 + a_17)
s_17 = (17/2) * (5 + (-59))
s_17 = (17/2) * (-54)
s_17 = 17 * (-54 / 2)
s_17 = 17 * (-27)
s_17 = -459

Двоечник!