Предмет: Алгебра, автор: Крис07071

решите плиз срочно!!!
пусть а5=9,а9=-3 - член арифметической прогрессии.найдите S7

Ответы

Автор ответа: Марал

1

a5=a1+4d
a9=a1+8d

$\left \{ {{9=a1+4d} \atop {-3=a1+8d}} \right.$
из второго уравнения отнимаем первое и получаем:-12=4d
d=-3
подставляем d в первое уравнение,чтобы найти a1:
$9=a1+4(-3)$
$9=a1-12$
a1=21
$S7= \frac{a1+a7}{2} *7$
a7=a1+6d=21+6(-3)=21-18=3
$S7= \frac{21+3}{2}*7= \frac{24}{2} *7=12*7=84$
ответ:84

Предыдущий вопрос

Следующий вопрос

Интересные вопросы

Предмет: Английский язык, автор: ArtemMontano

Откройте скобки.
1.What are you (to do) after work today? -I (to meet) Brown for a game of tennis.
2.Father is tired.He (to go ) to bed early tonight.
3.If I (to have ) enough time right now,I (to write) my friend a letter.
4.When Lisa (to arrive )?-She (to arrive) at six tomorrow.
5.Bella (to have) a very bad habit.She constantly (interrupt ) people.
6.When Dennis (to hear) the news,he (to be) very glad.
7.How do you spell "tomorrow"?-I (to look) it up in the dictionary for you.
8.If Sam (not to finish)work in time, he (to be ) late for his bus.
9.Now I (to see) what kind of person she (to be).
10.Look at Sandy !She (to look) angry.
11.As soon as he (to contact) us, we (to give) him your message.
12.Don't forget to take your jacket. It (to get ) colder.
13.I (to be) tired of having you for a roommate!You constantly (to take) my discs without asking.
14.He (to think) about his future right now.He (to think) it (to be) brilliant.
15.Now I (to know) the way of doing it.
16.Their trip (to start) tomorrow.They (to leave ) at 7.
17.Where you (to go)?-I (to be) back in a moment.
18.John (to be) busy now.He (to have) his guitar lesson at the moment.
19.I (not to like )Alan at all!He always (to giggle) when he (to see) me.
20.You (to see) these papers now.Could you cheek them?

2 года назад

Предмет: Английский язык, автор: GALILE

Сделайте задание 2) срочно надо пожалуйста)))))))))

2 года назад

Предмет: Русский язык, автор: gvggg

Решите пожалуйста по русском тест только под номером 3

2 года назад

Предмет: Английский язык, автор: sans2288

Можете провирить чи правильно я все вставив

7 лет назад

Предмет: История, автор: milashka4041

Приведите 10 примеров материальных источников
Пж плиззз 15 б

7 лет назад