Question

решите пж , даю 45 баллов

Answer 1

$a_{1} =12\\\\d=-4\\\\a_{8}=a_{1}+7d=12+7*(-4)=12-28=-16\\\\a_{20}=a_{1}+19d=12+19*(-4)=12-76=-64\\\\S_{13} =\frac{a_{8}+a_{20}}{2}*13=\frac{-16-64}{2}*13=-40*13=-520\\\\\boxed{S_{13}=-520}$