B3=25;b4=125 найти S5

Question

B3=25;b4=125 найти S5

Comments

Геометрическая?

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Да

Answer 1

B4 = b3*q
q = b4:b3 = 125:25 = 5
b3 = b1*q^2
b1 = b3:q^2 = 25:25 = 1

S(5) = b1(1-q^5)/(1-q) = 1*(1-3125)(1-5) = 3124/4 = 781