[tex]\displaystyle\bf\\S=8\\\\q=\frac{1}{2}\\\\\\S=\frac{b_{1} }{1-q} \\\\\\b_{1} =S\cdot (1-q)=8\cdot \Big(1-\frac{1}{2} \Big)=8\cdot\frac{1}{2} =4\\\\\\Otvet \ . \ b_{1}=4[/tex]