Question

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

Answer 1

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