Question

Геометрическая прогрессия

Answer 1

$b_1 = 4, b_5 = 324, q <0;\\b_5 = b_1*q^4\\q^4 = 81; q = -3\\S_n = \frac{b_1(q^n-1)}{q-1}\\S_6 = \frac{4(729-1)}{-4} = -728$