Предмет: Алгебра, автор: dedezh9586

Арифметическая прогрессия А4=20 а6=25 S4=?​

Ответы

Автор ответа: sangers1959
1

Ответ: S₄=65.

Объяснение:

\displaystyle\\\left \{ {{a_4=20} \atop {a_6=25}} \right. \ \ \ \ \ \ \left \{ {{a_1+3d=20\ \ \ \ \ \ (1)} \atop {a_1+5d=25\ \ \ \ \ \ (2)}} \right.

Вычитаем из уравнения (2) уравнение (1):

2d=5 |:1

d=2,5.

a₁+3*2,5=20

a₁+7,5=20

a₁=12,5.

\displaystyle\\S_4=\frac{12,5+20}{2}*4=32,5*2=65.

kimkarina953: Здравствуйте, не могли бы помочь с алгеброй пожалуйста
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