Question

Якщо в арифметичні прогресії (аn) a1=-4; d=7, то S20 дорівнює: Срочно!!! ДАЮ 40бал!

Answer 1

Розв’язання на фото

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Answer 2

Arithmetical Progression and Sum of Terms

In an arithmetic progression, the nth term (an) is given by the formula:

an = a1 + (n - 1)d

Where: - a1 = the first term - d = the common difference between the terms - n = the term number

Given: - a1 = -4 - d = 7 - n = 20

We can calculate the 20th term using the formula: a20 = a1 + (20 - 1)d

Let's calculate a20: a20 = -4 + (20 - 1) * 7 a20 = -4 + 19 * 7 a20 = -4 + 133 a20 = 129

Now that we have the 20th term (a20), we can calculate the sum of the first 20 terms (S20) using the formula: S20 = n/2 * (a1 + an)

Substitute the values: S20 = 20/2 * (-4 + 129) S20 = 10 * 125 S20 = 1250

Therefore, S20 = 1250.