Найдите сумму первых 20 членов арифметической .прогрессии.: -15; -11; -7 за ранее ♥ ! ♥

S(20)-?

a(1) = -15

a(2) = -11

d = -11 - (-15) = 4

a(20) = a(1) + 19d

a(20) = -15 + 19*4 = -15 + 76 = 61

S(20) = [a(1) +a(20)] / 2 * 20

S(20) = [a(1) + a(20)] * 10

S(20) = (-15+61) * 10 = 460