Предмет: Алгебра, автор: alexlipovec10

CРОЧНО Знайдіть суму всіх додатних членів арифметичної прогресії 4;3.3;2.6

Ответы

Автор ответа: hote

1

Арифметическая прогрессия

$\displaystyle a_1=4; a_2=3.3; a_3=2.6\\\\d=3.3-4=-0.7\\\\$

найдем количество положительных членов прогрессии

$\displaystyle a_n=a_1+d(n-1)\geq 0\\\\4-0.7(n-1)\geq 0\\\\4-0.7n+0.7\geq 0\\\\4.7\geq 0.7n\\\\n\leq 6\frac{5}{7}$

значит 6 членов будут положительными

$\displaystyle S_6=\frac{2a_1+d(n-1)}{2}*n=\frac{2*4-0.7*5}{2}*6=13.5$

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