Question

a18=-20 S10=-40 a1-?
АРИФМЕТРИЧНА ПРОГРЕСІЯ

Answer 1

$a_{18}=-20\\\\a_{1}+17d=-20\\\\a_{1} =-17d-20\\\\S_{10} =\dfrac{2a_{1} +9d}{2}\cdot 10\\\\S_{10} =(2a_{1}+9d)\cdot 5\\\\-40=(-17d-20+9d)\cdot 5\\\\-8d-20=-8\\\\-8d=12\\\\d=-1,5\\\\a_{1}=-17\cdot (-1,5)-20=25,5-20=5,5\\\\\boxed{a_{1}=5,5}$