B5 = b4 · q → q = 1/64 : 1/16 = 1/4
bn =b1 · q^(n -1)
n = 5
b5 = b1 · q^4 → b1 = 1/64 : (1/4)^4 = 1/64 · 256 = 4
Sn = b1 · (q^n - 1) / (q - 1)
n = 5
S5 = (b5 · q - b1) / (q - 1)
S5 = (1/64 · 1/4 - 4) / (1/4 - 1) = 341/64 = 5 21/64 = 5,328125
Ответ: 5 21/64 или 5,328125