Question

12+15+18+21+...+255=?​

Answer 1

Ответ:

10947

Пошаговое объяснение:

Это алгебпаическая прогрессия.

A1 = 12; Нужно найти номинацию числа 255.

An = A1 + d(n-1); 255 = 12 + 3(n-1); n = 82

Sn(сумма алгебраической прогрессии) = A1 + A82 / 2 * n; 12 + 255 / 2 * 82 = 10947

Ответ: 10947

Answer 2

The given sequence is an arithmetic progression where the common difference is 15 - 12 = 3. The first term is 12, and the last term is 255. To find the sum of this arithmetic series, you can use the formula for the sum of an arithmetic progression:

$S = \frac{n}{2} \cdot (a_1 + a_n)$

Where:

• $S$ is the sum of the series.
• $n$ is the number of terms.
• $a_1$ is the first term.
• $a_n$ is the last term.

In this case, $a_1 = 12$, $a_n = 255$, and we need to find $n$.

Using the formula to find $n$:

$a_n = a_1 + (n - 1) \cdot d$ $255 = 12 + (n - 1) \cdot 3$ $255 - 12 = 3n - 3$ $243 = 3n$ $n = 81$

Now that we have $n$, we can plug it into the formula for the sum:

$S = \frac{81}{2} \cdot (12 + 255)$ $S = 40.5 \cdot 267$ $S = 10753.5$

So, the sum of the sequence $12+15+18+21+...+255$ is $10753.5$.