Question

алгебра....
расписать
...

Answer 1

$b_1 = 11, b_4 = 88\\ frac{b_4}{b_1} = q^3, frac{88}{11} = 8 = 2^3 = q^3, q = 2\\ S_n = b_1 + ... + b_n = b_1 + b_1*q + ... + b_1*q^{n - 1} =\\ =b_1(1 + q + ... + q^{n - 1}) = b_1(frac{q^n - 1}{q - 1})\\ S_5 = 11frac{2^5 - 1}{2 - 1}= 11(32-1) = 11*31 = boxed{341}$