Sum of the First n Terms of an AP

Oct 13, 2025 by talynt.org 89 views

The Sum of the First 7 Terms and the 17th Term of an Arithmetic Progression (AP)

In this article, we will explore the concept of an arithmetic progression (AP) and how to find the sum of its first n terms and the nth term itself. We will use the given information about the sum of the first 7 terms and the 17th term to find the first term and the common difference of the AP.

What is an Arithmetic Progression (AP)?

An arithmetic progression (AP) is a sequence of numbers in which each term after the first is obtained by adding a fixed constant to the previous term. This fixed constant is called the common difference (d). The general form of an AP is:

a, a + d, a + 2d, a + 3d, ...

where a is the first term and d is the common difference.

Sum of the First n Terms of an AP

The sum of the first n terms of an AP can be calculated using the formula:

S_n = n/2 [2a + (n-1)d]

where S_n is the sum of the first n terms, a is the first term, d is the common difference, and n is the number of terms.

Given Information

We are given that the sum of the first 7 terms of the AP is 49 and the sum of the first 17 terms is 249. We can use this information to set up two equations:

S_7 = 49 S_17 = 249

Using the Formula to Set Up Equations

We can use the formula for the sum of the first n terms to set up two equations:

7/2 [2a + (7-1)d] = 49 17/2 [2a + (17-1)d] = 249

Simplifying these equations, we get:

7(2a + 6d) = 98 17(2a + 16d) = 498

Solving the Equations

We can solve these equations simultaneously to find the values of a and d.

First, we can simplify the equations further:

14a + 42d = 98 34a + 272d = 498

Next, we can multiply the first equation by 34 and the second equation by 14 to eliminate the variable a:

476a + 1428d = 3332 476a + 3792d = 6972

Subtracting the first equation from the second equation, we get:

1364d = 3640

Dividing both sides by 1364, we get:

d = 2.67

Now that we have found the value of d, we can substitute it into one of the original equations to find the value of a. We will use the first equation:

14a + 42(2.67) = 98

Simplifying this equation, we get:

14a + 112.74 = 98

Subtracting 112.74 from both sides, we get:

14a = -14.74

Dividing both sides by 14, we get:

a = -1.05

Finding the 17th Term

Now that we have found the values of a and d, we can find the 17th term of the AP using the formula:

a_n = a + (n-1)d

Substituting the values of a and d, we get:

a_17 = -1.05 + (17-1)(2.67) = -1.05 + 16(2.67) = -1.05 + 42.72 = 41.67

In this article, we have used the given information about the sum of the first 7 terms and the 17th term of an AP to find the first term and the common difference of the AP. We have also used the formula for the sum of the first n terms to set up equations and solve for the values of a and d. Finally, we have found the 17th term of the AP using the formula for the nth term.

[1] Khan Academy. (n.d.). Arithmetic Progression. Retrieved from https://www.khanacademy.org/math/algebra2/x2f4f4d6/x2f4f4d7
[2] Math Open Reference. (n.d.). Arithmetic Progression. Retrieved from https://www.mathopenref.com/arithmeticprogression.html

In our previous article, we explored the concept of an arithmetic progression (AP) and how to find the sum of its first n terms and the nth term itself. We also used the given information about the sum of the first 7 terms and the 17th term to find the first term and the common difference of the AP. In this article, we will answer some frequently asked questions about AP to help you better understand this concept.

Q: What is an arithmetic progression (AP)?

A: An arithmetic progression (AP) is a sequence of numbers in which each term after the first is obtained by adding a fixed constant to the previous term. This fixed constant is called the common difference (d).

Q: What is the formula for the sum of the first n terms of an AP?

A: The formula for the sum of the first n terms of an AP is:

S_n = n/2 [2a + (n-1)d]

where S_n is the sum of the first n terms, a is the first term, d is the common difference, and n is the number of terms.

Q: How do I find the first term (a) and the common difference (d) of an AP?

A: To find the first term (a) and the common difference (d) of an AP, you can use the given information about the sum of the first n terms and the nth term. You can set up equations using the formula for the sum of the first n terms and solve for the values of a and d.

Q: What is the formula for the nth term of an AP?

A: The formula for the nth term of an AP is:

a_n = a + (n-1)d

where a_n is the nth term, a is the first term, d is the common difference, and n is the term number.

Q: Can I use the formula for the nth term to find the first term (a) and the common difference (d) of an AP?

A: No, you cannot use the formula for the nth term to find the first term (a) and the common difference (d) of an AP. The formula for the nth term requires you to know the values of a and d, which you can find using the formula for the sum of the first n terms.

Q: What is the difference between an arithmetic progression (AP) and a geometric progression (GP)?

A: An arithmetic progression (AP) is a sequence of numbers in which each term after the first is obtained by adding a fixed constant to the previous term. A geometric progression (GP) is a sequence of numbers in which each term after the first is obtained by multiplying the previous term by a fixed constant.

Q: Can I use the formula for the sum of the first n terms of an AP to find the sum of the first n terms of a GP?

A: No, you cannot use the formula for the sum of the first n terms of an AP to find the sum of the first n terms of a GP. The formula for the sum of the first n terms of a GP is different from the formula for the sum of the first n terms of an AP.

Q: What are some real-life applications of arithmetic progressions (AP)?

A: Arithmetic progressions (AP) have many real-life applications, including:

Finance: APs are used to calculate interest rates and investment returns.
Music: APs are used to create musical scales and melodies.
Science: APs are used to model population growth and decay.
Engineering: APs are used to design and optimize systems.

In this article, we have answered some frequently asked questions about arithmetic progressions (AP) to help you better understand this concept. We have also provided examples and explanations to help you apply the formulas and concepts to real-life situations.

[1] Khan Academy. (n.d.). Arithmetic Progression. Retrieved from https://www.khanacademy.org/math/algebra2/x2f4f4d6/x2f4f4d7
[2] Math Open Reference. (n.d.). Arithmetic Progression. Retrieved from https://www.mathopenref.com/arithmeticprogression.html

The answers to the questions are based on the formulas and concepts discussed in the previous article. If you have any further questions or need clarification on any of the concepts, please feel free to ask.