Question

ПОМОГИТЕ ПОЖАЛУЙСТА

Answer 1

Ответ:

$a_5=6\ ,\ a_9=15\\\\a_5=a_1+4d\ \ ,\ \ \ a_9=a_1+8d\\\\\\\left\{\begin{array}{l}a_1+4d=6\\a_1+8d=15\end{array}\right\ \ \left\{\begin{array}{l}a_1=6-4d\\a_1=15-8d\end{array}\right\ \ \left\{\begin{array}{l}6-4d=15-8d\\a_1=6-4d\end{array}\right$

$\left\{\begin{array}{l}4d=9\\a_1=6-4d\end{array}\right\ \ \left\{\begin{array}{l}d=\dfrac{9}{4}\\a_1=6-4\cdot \dfrac{9}{4}\end{array}\right\ \ \left\{\begin{array}{l}s=\dfrac{9}{4}\\a_1=-3\end{array}\right\\\\\\Otvet:\ \ a_1=-3$