Объяснение:

см фото,,,,,,,,,,,,,,,,,,,,,,,,,,

[tex]\displaystyle\bf\\q=-2\\\\S_{4} =65\\\\b_{1} =?\\\\\\S_{4}=\frac{b_{1} \cdot(1-q^{4} )}{1-q} \\\\\\65=\frac{b_{1} \cdot\bigg[1-(-2)^{4} \bigg]}{1-(-2)} \\\\\\65=\frac{b_{1} \cdot(1-16)}{1+2} \\\\\\65=\frac{b_{1} \cdot(-15)}{3} \\\\\\-5b_{1} =65\\\\\\\boxed{\boxed{b_{1} =-13}}[/tex]