Question

СРОЧНО!Даю 40 баллов!Без плагиата!

Answer 1

$a_3=14\; ,\; \; a_8=-1\\\\a_3=a_1+2d\; \; ,\; \; a_8=a_1+7d\\\\\left \{ {{a_1+2d=14} \atop {a_1+7d=-1}} \right. \; \left \{ {{14-2d=-1-7d} \atop {a_1=-1-7d}} \right. \; \left \{ {{5d=-15} \atop {a_1=-1-7d}} \right. \; \left \{ {{d=-3} \atop {a_1=20}} \right. \; \; \Rightarrow \; \; a_1=20\; ,\; \; d=-3\\\\\\S_{10}=\frac{2a_1+d(10-1)}{2}\cdot 10=\frac{40-3\, \cdot \, 9}{2}\cdot 10=\frac{13}{2}\cdot 10=65$