Question

Найдите s5, если a1=64 q=1/4

Answer 1

$S_n= dfrac{a_1cdot(1-q^n)}{1-q} \ \ \ S_5= dfrac{a_1cdot(1-q^5)}{1-q} = dfrac{64cdot(4^5-1)}{4^5-4^4} = dfrac{4^3(4^5-1)}{4^3(4^2-4)} = dfrac{4^5-1}{4^2-4} =85.25$