Sequence 21, 17, 13, 9...

Sequence 21, 17, 13, 9,...

Here we answer many questions about the sequence 21, 17, 13, 9... What type of sequence is 21, 17, 13, 9? What is the next number in the sequence 21, 17, 13, 9? What is the nth number in the sequence 21, 17, 13, 9? What is the sum of the first 20 terms in the sequence 21, 17, 13, 9? What is the sum of the first n numbers in the sequence 21, 17, 13, 9?

In addition, we will also give you the formula that is used to calculate the next number or the nth number in 21, 17, 13, 9, and the formula to calculate the sum of n numbers in 21, 17, 13, 9.

A sequence is a list of numbers in a pattern, and each number in the sequence is called a term. We will use "terms" and "numbers" interchangeably on this page.

So, what type of sequence is 21, 17, 13, 9? The sequence 21, 17, 13, 9 has a common difference of -4 between each term. We call this kind of sequence an arithmetic sequence. Below is an image illustrating the correlation between the arithmetic sequence 21, 17, 13, 9 and its common difference of -4.

Sequence 21, 17, 13, 9,...

Now, what is the next number in the sequence 21, 17, 13, 9? Below is the formula used to calculate the next number in an arithmetic sequence, such as 21, 17, 13, 9. The first term listed in the sequence is "a", the common difference is "d", and "n" is the nth term of the arithmetic sequence.

a + (n-1) × d = Next Term

As stated above, the common difference (d) between each term is -4. Furthermore, the next term in 21, 17, 13, 9 is the fifth term (5), and the first term is 21. When we enter these values into our formula, we get the following answer:

21 + (5 - 1) × -4 = 5

Thus, the next number (term) in the sequence 21, 17, 13, 9 is 5. The tool below calculates the nth term of the sequence using the formula above. For example, type in 20 if you want the 20th term in the sequence 21, 17, 13, 9, or 100 if you want the 100th term in the sequence 21, 17, 13, 9.

Let's move on to our next question. What is the sum of the first 20 terms in the sequence 21, 17, 13, 9? We use the formula below to calculate the sum of the first n terms in an arithmetic sequence such as 21, 17, 13, 9. Again, note that the first term is "a", the common difference is "d", and "n" is the nth term of the arithmetic sequence.

(n/2)((2 × a) + (n - 1) × d) = Sum

When we enter the a, d, and n values into our formula, where n is equal to 20, we can calculate the sum of all numbers up through the 20th term in the sequence 21, 17, 13, 9, as seen below:

(20/2)((2 × 21) + (20 - 1) × -4) = -340

Therefore, the sum of all numbers up through the 20th term in the sequence 21, 17, 13, 9 is -340. Below is another tool we created to make these calculations easier for you. This tool can calculate the sum of any number of terms in the sequence 21, 17, 13, 9 using the formula mentioned above. For example, if you type in 50, then it will calculate the sum of the first 50 terms in the sequence 21, 17, 13, 9.

Arithmetic Sequence Calculator
Go here to learn more about arithmetic sequences using the best online Arithmetic Sequence Calculator.

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