Предмет: Алгебра, автор: asikazaitofa536

СРОЧНО ПОМОГИТЕ ОЧЕНЬ НАДО ПОЖАЛУЙСТА!!!!!!

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Ответы

Автор ответа: Вика240

A) Sn=(a1+an)n/2
-21=(15+a7)7/2
-42=(15+a7)7
-6=15+a7
a7=-21

Б) -20=(а1-30)10/2
-20=(а1-30)5
-4= а1-30
а1=26

В) 26=7+1n-1
n=20

Г)67=a1+2/3*99
67=a1+66
a1=1

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