Ответ:

[tex] b1 = - 3\: \: \: \: \: b2 = 15 \\ b3 = b1 \times {q}^{n - 1} \\ q = ? \\ b2 = b1 \times {q}^{2 - 1} \\ 15 = - 3 \times q \\ q = - 5 \\ b3 = - 3 \times {( - 5)}^{3 - 1} \\ b3 = - 3 \times ( - 5 {)}^{2} \\ b3 = - 3 \times 25 \\ b3 = - 75 \\ s5 = \frac{b1 ( {q}^{n} - 1) }{q - 1} \\ s5 = \frac{ - 3( - 75 {}^{3} - 1)}{ - 75 - 1} \\ s5 = \frac{ - 3( - 421875 - 1)}{ - 75 - 1} \\ s5 = \frac{ - 3 \times ( - 421876)}{ - 76} \\ s5 = \frac{1265628}{ - 76} \\ s5 = 16653[/tex]

Ответ: b3=-75; S5=16653