Question

найдите сумму первых шести членов геометрической прогрессии b1=1 b4=8

Answer 1

b4 = b1*q^3 = q^3 = 8

q^3 = 8
q^3 = 2^3
q = 2

S6 = (b1*(1 - q^6))/( 1 - q) = (1 - q^6)/(1 - q)  = 
= ( 1 - 2^6) / (1 - 2) = (2^6 - 1)/ (2 - 1)  = 2^6 - 1 = 63