Предмет: Математика, автор: Magaaa

дана арифметическая прогрессия {аn} вычислите сумму 5 членов, если а7=-30, d=-4

Ответы

Автор ответа: DisRah

Sn=(a1+an)n/2
a7=a1+6d a1=a7-6d=-30+24=-6
an=a1+d(n-1) a5=a1+4d=-6+(-4*4)=-6-16=-22
S5=(-6-22)*5/2=-140/2=-70

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