Предмет: Алгебра, автор: AlHeyden

Помогите, пожалуйста, алгебра, 9 класс

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Автор ответа: УбийцаВремени

аn+1=аn-10
а1+1=а1-10
a2=a1-10
a2=-15-10
a2=-25
a3=a2-10
a3=-25-10
a3=-35
d=a3-a2=-35-(-25)=-10
S8=((2a1+d(n-1))/2)*n
S8=((-30+7*(-10))/2)*8=-50*8=-400

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