Question

ребята помогите пожалуйста
Тока задание 3!!! ​

Answer 1

$\displaystyle\bf\\\left \{ {{a_{7} =18} \atop {a_{3} =22}} \right. \\\\\\-\left \{ {{a_{1} +6d=18} \atop {a_{1} +2d=22}} \right.\\---------\\4d=-4\\\\d=-1\\\\a_{1} =22-2d=22-2\cdot(-1)=22+2=24\\\\a_{9} =a_{1} +8d=24+8\cdot(-1)=24-8=16\\\\\\a_{8} =\frac{a_{7} +a_{9} }{2} =\frac{18+16}{2} =\frac{34}{2} =17$