Question

Геометрическая прогрессия b2=4 b4=1 найтм s6

Answer 1

b4 = b2 * q^2
1 = 4 * q^2
q = 1/2    или  - 1/2
Если q = 1/2 :
b1 = b2/q  = 8
sn = b1*(1-q^n)/(1-q) 
s6 = 8*(1-1/64)/(0.5) = 8*63 / (64*0.5) = 63/4 = 15.75
Если q = -1/2 :
b1 = b2/q  = -8
sn = b1*(1-q^n)/(1-q) 
s6 = -8*(1-1/64)/(1.5) = -8*63 / (64*1.5) = -63/12 = -5,25