Question

Найти a17 и d,если a16=4,a18=-12

Answer 1

$a_{16}=4$
$a_{18}=-12$
$\left \{ {{a_{16}=a_{1}*15d=4} \atop {a_{18}=a_{1}*17d=-12}} \right.$
-2d=16
d=16:(-2)
d=-8
$a_{1}=a_{16}-15d=4+120=124$
$a_{17}=a_{1}+16d=124-128=-4$