Question

Помогите пж срочно срочно

Answer 1

$b_{1}=81\\\\\ q=\frac{1}{3}\\\\S_{5} =\frac{b_{1}*(1-q^{5})}{1-q}=\frac{81*[1-(\frac{1}{3})^{5}]}{1-\frac{1}{3}}=\frac{81*(1-\frac{1}{243}) }{\frac{2}{3}} =\frac{81*3*\frac{242}{243} }{2}=\frac{242}{2}=121\\\\\boxed{S_{5}=121}$