Question

Найдите первый член арифметической прогрессии

Answer 1

$\left \{ {{a_{15} =38} \atop {a_{8} =17}} \right.\\\\-\left \{ {{a_{1}+14d=38 } \atop {a_{1 }+7d=17} \right.\\\\--------\\\\7d=21\\\\a_{1}+7d=17\\\\a_{1}=17-7d=17-21=-4$