(bn) :q=2 n=7 S7=635 знайти b1 b7

Question

(bn) :q=2 n=7 S7=635 знайти b1 b7

Answer 1

Сумма
S = b1 * (1-q^n) / (1-q)
S7 = b1 * (1-2^7) / (1-2) = 635
b1 * 127 = 635
b1 = 5
Шестой член
b6 = b1 * q^5 = 5 * 2^5 = 160

---