B₈=5/3¹²;b₁₄=5/3⁶;S₁₁=?;
b₈=b₁q⁷;
b₁₄=b₁q¹³;
b₁₄/b₈=q¹³⁻⁷=q⁶;
q=⁶√(5/3¹² :5/3⁶)=⁶√(1/3⁶)=1/3;
b₁=b₈/q⁷=5/3¹² :1/3⁷=5/3¹²⁻⁷=5/3⁵;
S₁₁=b₁(1-q¹¹)/(1-q);
S₁₁=5/3⁵·(1-1/3¹¹)/(1-1/3)=5/3⁵·(3¹¹-1)/3¹¹ :(2/3)=
=5·(3¹¹-1)·3/3⁵·3¹¹·2=5·(3¹¹-1)·3¹⁻⁵⁻¹¹·2⁻¹=5·(3¹¹-1)·3⁻¹⁵2⁻¹;