найти сумму 20 первых членов арифметической прогрессии: 5;2;-1;-4;... - №12650251

найти сумму 20 первых членов арифметической прогрессии: 5;2;-1;-4;...
- Предмет:
  Математика
- Автор:
  kassandrajhnh
- 6 лет назад

Ответы 1

$a_{n}$ = a1 + d(n-1)d = 2 - 5 (или) -1 - 2 (или) -4 - (-1) = -3a20 = a1 + d(20-1) = 5 + (-3)*19 = - 52S = (a1+ $a_{n}$ ) : 2 * n = (5 + (-52)) : 2 * 20 = -470
- Автор:
  simeóndbr3
- 6 лет назад
- 0
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