Question

Поооооомогите!!!Пожалуйста ​

Answer 1

$1)b_{1} =27\\\\q=\frac{1}{3}\\\\b_{5} =b_{1}*q^{4}=27*(\frac{1}{3} )^{4}=\frac{3^{3} }{3^{4}}=\frac{1}{3}\\\\S_{4}=\frac{b_{1}(1-q^{4} )}{1-q} =\frac{27*(1-\frac{1}{81}) }{1-\frac{1}{3} }=\frac{27*\frac{80}{81} }{\frac{2}{3} }=\frac{80*3}{2*3} =40$

$2)b_{1}=28\\\\b_{2}=-14\\\\b_{2}=b_{1}*q\\\\q=\frac{b_{2}}{b_{1}}=\frac{-14}{28}=-\frac{1}{2}\\\\S=\frac{b_{1}}{1-q}=\frac{28}{1-(-\frac{1}{2}) }=\frac{28}{1+\frac{1}{2} }=\frac{28}{\frac{3}{2} }=\frac{28*2}{3}=18\frac{2}{3}$