Arithmetic Sequence: Finding Difference Between 24th and 21st Terms

Question

What is the difference between the value of the 24th element and the value of the 21st element of the sequence below?

2,5,8 2,5,8\ldots

Video Solution

Solution Steps

00:00 Find the difference between terms 24 and 21
00:06 Let's observe the constant difference between terms
00:17 Let's count how many terms are between 24 and 21
00:41 We can see that the difference between the numbers equals 3 times the difference
00:48 And this is the solution to the question

Step-by-Step Solution

Given the sequence 2,5,8,2, 5, 8, \ldots, we will find the difference between the 24th and 21st terms.

First, identify the parameters of the sequence:
The first term a1=2a_1 = 2.
The common difference dd is 52=35 - 2 = 3.

The formula for the nth term of an arithmetic sequence is given by:

an=a1+(n1)da_n = a_1 + (n-1)d

Let's find the 24th term:

a24=a1+(241)d=2+233a_{24} = a_1 + (24-1) \cdot d = 2 + 23 \cdot 3

a24=2+69=71a_{24} = 2 + 69 = 71

Next, find the 21st term:

a21=a1+(211)d=2+203a_{21} = a_1 + (21-1) \cdot d = 2 + 20 \cdot 3

a21=2+60=62a_{21} = 2 + 60 = 62

Now calculate the difference between these terms:

a24a21=7162=9a_{24} - a_{21} = 71 - 62 = 9

Therefore, the difference between the 24th and 21st terms of the sequence is 9.

Answer

9