Question

В арифметической прогрессии (aₙ):a₂=-8; a₉=27. Найдите a₅​

Answer 1

$a_2=-8\; ,\; \; a_9=27\\\\\\\left\{\begin{array}{l}a_1+d=-8\\a_1+8d=27\end{array}\right\; \; \left\{\begin{array}{l}-8-d=27-8d\\a_1=27-8d\end{array}\right\; \; \left\{\begin{array}{l}7d=35\\a_1=27-8d\end{array}\right\; \; \left\{\begin{array}{l}d=5\\a_1=27-40=-13\end{array}\right\\\\\\a_5=a_1+4d=-13+4\cdot 5=7$